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STUDIES  OF 


INHERITANCE  IN  RABBITS 


W.   E.   CASTLE 


IN  COLLABORATION  WITH 

H.  E.  WALTER,  R.  C.  MULLENIX,  and  S.  COBB 


WASHINGTON,  D.  C. 

Published  by  The  Carnegie  Institution  of  Washington 

1909 


Carnegie  Institution  of  Washington,  Publication  No.  114. 


Papers  of  the  Station  for  Experimental  Evolution,  No.  13. 


Contributions  from  the  Zoological  Laboratory  of  the  Museum 

of  Comparative  Zoology  at  Harvard  College. 

E.  L.  Mark,  Director.     No.  199. 


The  Plimpton  Press  Norwood  Mass.  U.S.A. 


CONTENTS. 


Kagt 

Preface    5 

Part  I.  —  Ear-size 9-35 

Introduction 9 

Characteristics  of  lop-eared  rabbits;  sterility  and  its  inheritance 9 

Growth-rate  of  lop-eared  and  of  short-eared  rabbits  in  size  and  in  ear-length      .  la 

Matings  of  short-eared  rabbits  iuler  se 14 

Matings  of  lop-eared  rabbits  inter  se 16 

Cross  I.  —  Lop-eared  female  X  short-eared  male 18 

Cross  2.  —  Short-eared  female  X  lop-eared  male 18 

Cross  3.  —  Belgian  hare  female  X  lop-eared  male 20 

Cross  4.  —  Lop-eared  female  X  half-blood  lop  male 21 

Cross  5.  —  Half-blood  lop  female  X  lop  male  179 22 

Cross  6.  —  Half-blood  lops  mated  ititer  se 23 

Other  matings  of  half-blood  lops  and  additional  Cross  6  matings 26 

Matings  of  three-quarter-blood  lops 3I 

Cross  7.  —  Quarter-blood  lop  female  X  short-eared  male 3i 

Cross  8.  —  Quarter-blood  lop  X  three-quarter-blood  lop 34 

Limitations  of  the  data  studied 34 

Conclusions 35 

Part  II.  —  Weight 37-4° 

Part  III.  —  Skeletal  Dimensions        4i-44 

Part  IV.  —  Color 45-68 

Color  variation  in  relation  to  color  factors 45 

Development  of  the  factor  hypothesis 46 

The  general  color  factor,  C 46 

The  specific  pigment  factors,  B,  Br,  and  Y 46 

The  intensity  factor,  I  or  D 46 

The  factor  for  a  pigment  pattern  of  the  individual  hair,  A 47 

The  factor  for  uniformity  of  pigmentation,  U,  or  spotting  with  white,  S     .      .  47 
The  factor  for  extended  distribution  of  black  or  brown,  E,  alternative  with  R 

(restricted  distribution)         47 

Interrelations  of  factors  E  and  U 48 

Interrelations  of  factors  B,  Br,  and  V 49 

Gametic  structure  and  variation 49 

Gametic  and  zygotic  formulae 5° 

Color  varieties  of  the  rabbit 5' 

Gray  type 5  2 

Black  type 5^ 

Yellow  type 53 

White  type 53 

Zygotic  variation  within  each  color  variety 54 

Gray 54 

Blue-gray 60 

Black 61 

Blue 6a 

Yellow 63 

Sooty 64 

White 65 

The  material  basis  of  heredity  factors 68 

Bibliography 69 

Description  of  Plates 7° 


3 


PREFACE. 


In  this  paper  arc  recorded  observations  upon  inheritance  in  rabbits 
which  were  made  in  the  Harvard  Zoological  Laboratory  with  the  aid  of  a 
grant  from  the  Carnegie  Institution  of  Washington.  The  authors  desire 
to  express  their  appreciation  of  that  aid,  without  which  these  observations 
could  not  have  been  made. 

The  experiments  described  were  planned  by  the  senior  author,  and 
this  report  also  was  written  by  him.  Dr.  H.  E.  Walter  has  made  the 
majority  of  the  extremely  laborious  observations  and  computations  con- 
cerning the  inheritance  of  ear-length  and  of  body-weight.  Dr.  R.  C. 
MuUenix  prepared  and  measured  the  rabbit  skeletons  as  a  foundation  for 
Part  III  of  this  paper;  while  both  Dr.  Walter  and  Mr.  Cobb  rendered 
valuable  assistance  in  connection  with  the  study  which  has  been  made  of 
color  inheritance.  The  senior  author  alone  is  responsible  for  the  analytical 
treatment  of  the  observations. 


STUDIES    OF   INHERITANCE    IN    RABBITS 


BY   W.    E.    CASTLE 


IN  COLLABORATION  WITH 


H.    E.    WALTER,    R.    C.    MULLENIX,    AND    S.    COBB. 


PART  I.  — EAR-SIZE. 


INTRODUCTION. 


The  inheritance  of  ear-size  in  rabbits  has  been  characterized  as  blend- 
ing, in  certain  preliminary  publications,  (Castle,  :o5,  :05a).'  The  experi- 
mental evidence  for  such  a  characterization  is  described  in  the  following 
pages.  It  consists  of  results  obtained  by  experimental  cross-breeding  of 
lop-eared  rabbits  with  ordinary  short-eared  ones.  A  detailed  account 
of  this  evidence  is  of  little  interest  to  the  general  reader,  who  therefore 
may  advantageously  omit  pages  14-34.  For  consultation  on  the  part  of 
the  critical  student  of  heredity,  it  has  been  thought  essential  to  present  this 
evidence  in  some  detail,  even  though  it  is  intrinsically  uninteresting. 

CHARACTERISTICS    OF    LOP-EARED    RABBITS;    STERILITY    AND    ITS 

INHERITANCE, 

Lop-eared  rabbits  are  distinguished  from  ordinary  ones  chiefly  by  the 
enormous  size  of  their  ears,  which  are  so  large  as  to  hang  down,  touching 
the  ground  on  either  side  of  the  head.  (See  plate  i,  fig.  2,  and  plate  2, 
fig.  8.)  This  breed  of  rabbit  is  characterized  also  by  a  long  tail  and 
unusual  size,  being  one  of  the  largest  breeds  known.  The  characters  of 
large  size  and  long  tail,  however,  have  probably  not  been  sought  for  their 
own  sake,  but  have  been  incidentally  obtained  in  the  production  of  the 
breed  as  a  result  of  selection  for  ears  of  large  size;  for  among  lop-eared 
rabbits,  as  a  rule,  those  of  the  largest  size  have  longest  ears. 

In  the  winter  of  1904  a  pair  of  lop-cared  rabbits  was  obtained  from  a 
fancier  and  used  in  various  breeding  experiments.  Matings  of  the  two 
together  were  for  the  most  part  fruitless,  only  one  litter  of  2  young  being 
successfully  reared.  These  were  similar  to  the  parent  rabbits  in  size  and 
ear-character.  Of  the  two,  one  was  a  male,  which  was  used  extensively 
in  breeding  experiments,  including  one  successful  mating  with  his  mother, 
from  which  came  a  good-sized  litter.  But  only  two  out  of  this  litter  at- 
tained the  age  of  20  weeks  and  they  ultimately  succumbed  to  disease  under 
conditions  not  unfavorable  to  other  rabbits.  The  second  of  the  two  young 
reared  by  the  original  lop-eared  pair  was  a  female.  Only  twice  did  this 
rabbit  bear  young  by  any  sort  of  mating.  In  one  case  she  failed  to  rear 
any  of  the  young.     In  the  other  case  she  reared,  when  mated  to  her  own 

•  For  complete  titles  see^Bibliography,  p..  69. 


■'■  '■'-.-TO  T"^ 


10  INHERITANCE    IN   RABBITS 

brother,  2  young  out  of  a  litter  of  3.  Both  were  males.  The  larger  one, 
although  apparently  healthy,  failed  to  breed;  the  smaller  one  was  not 
tested.  Accordingly,  out  of  5  pure-bred  lop-eared  rabbits  with  which  we 
have  experimented,  2  (a  male  and  a  female)  were  infertile  —  one  of  the 
two  largely  so,  and  the  other  completely.  Infertility  has  also  been 
encountered  among  a  few  of  the  female  descendants  of  this  lop-eared  stock 
produced  by  cross-breeding,  but  in  no  other  stock  of  rabbits  with  which 
we  have  experimented.  Sterile  individuals  have  not  been  observed  among 
half-blood  lops  of  generation  Fi,  but  a  few  have  occurred  in  later  genera- 
tions. In  the  majority  of  cases,  however,  the  sterile  individuals  have  been 
three-quarter-blood  lops. 

From  these  facts  we  conclude  that  a  tendency  to  sterility  is  inherent  in 
the  lop-eared  stock  used,  and  is  transmitted,  not  to  the  immediate  off- 
spring (FJ  if  they  are  cross-breds,  but  to  the  next  generation,  when  it 
is  produced  by  a  back-cross  between  Fj  and  the  pure  lop-eared  stock; 
less  frequently  sterility  reappears  in  F2,  produced  by  breeding  the  half- 
blood  lops  inter  se.  We  should  expect  the  infertility  to  occur  only  half 
as  frequently  in  this  latter  sort  of  mating  as  in  the  former,  where  it  has 
been  oftenest  observed.  On  the  whole,  it  seems  probable  that  a  ten- 
dency to  sterility  is  inherited  in  rabbits,  as  in  Drosophila  (see  Castle  et  al., 
:o6),  after  the  manner  of  a  Mendelian  recessive  character,  i.  e.,  skipping  a 
generation  in  crosses. 

Why  lop-eared  rabbits  more  than  other  breeds  should  show  a  tendency 
to  sterility  is  not  known;  but  as  they  are  extensively  inbred,  it  seems  highly 
probable  that  inbreeding  is  largely  responsible  for  this  sterihty.  The 
lop-eared  character  is  one  which,  from  the  manner  of  its  inheritance,  we 
may  be  sure,  has  been  built  up  slowly  as  the  result  of  selection.  In  this 
process  inbreeding  must  have  been  continuously  practised,  for  since  every 
out-cross  would  result  in  loss  of  half  the  ground  gained  by  selection,  it 
would  be  practised  only  when  absolutely  necessary. 

At  birth  rabbits  have  ears  quite  undeveloped,  and  the  ears  do  not  attain 
their  full  growth  until  an  age  of  5  to  8  months  have  been  reached.  Ear- 
growth  is  well  advanced,  however,  at  20  weeks,  after  which  time  it  becomes 
very  slow.  Accordingly  20  weeks  has  been  found  a  convenient  age  at 
which  to  institute  comparisons  as  to  ear-character  between  different  lots 
of  rabbits.  Frequently,  however,  it  is  impossible  to  rear  an  entire  Utter  of 
rabbits  to  the  age  of  20  weeks,  in  which  case  an  earlier  determination 
of  ear-character  becomes  desirable.  For  this  reason,  after  some  experi- 
mentation, we  adopted  the  plan  of  making  weekly  measurements  of  the 
ear  dimensions  at  ages  from  2  to  20  weeks  inclusive.  This  process,  while 
laborious,  fully  eliminates  errors  due  to  observation,  as  well  as  those  due 
to  temporary  growth  conditions. 

The  weekly  observations  upon  each  rabbit  included  taking  its  weight, 
the  maximum  length  and  maximum  width  of  its  right  ear,  and  finally  the 


EAR-SIZE 


11 


spread  of  the  ears,  i.  e.,  the  distance  from  ear-tip  to  ear-tip  when  the  ears 
are  extended  in  a  horizontal  position  and  stretched  slightly.  Since  the 
measurements  in  nearly  all  cases  were  made  by  the  same  observer  (Walter), 
the  personal  equation  is  a  fairly  constant  factor  and  may  be  disregarded 

Ear  length 

in  mm.  /      a    3     4     5     6     7     8     9    10    II    12    l^    14-    fS   I6    I7    id  19 

'      ' I I I '       I      I      I       I      »      I      I       I       I       I       I       I 


120 


110- 


100 


SO- 


SO- 


70 


60- 


50- 


40 


30 


JL 


,0 


o_-a' 


0--0841 
84-4 


y,        •' 1 1 1 1 r 1 1 1 1 1       "I 1 1        I        I        I        I        I        I 

Agem     ,       2      3      4       S      6      7      B      S     10     II     IZ     13     14     IS     I6     I7    JQ     19 

weeks. 

Fig.  I.   Chart  showing  growth  in  ear-length,  and  body-weight  of  a  litter  of  six  short-eared 
rabbits  between  the  ages  of  two  and  eighteen  weeks.      See  table  i. 


Weight 

in 
grams. 

■1500 
■1400 

1300 

1200 

1100 

1000 

Ysoo 

800 
700 
600 

\-500 
400 

300 
200 

100 


12  INHERITANCE    IN    RABBITS 

in  comparing  one  set  of  observations  with  another.  Records  of  this  sort, 
more  or  less  complete,  were  made  for  70  dilYerent  litters  of  rabbits, 
containing  341  individuals. 

An  inspection  of  figs,  i  to  3  shows  that  the  growth-curve  for  ear-length  ^ 
from  2  weeks  after  birth  is  of  the  same  general  form  in  the  case  of  both 
long-eared  and  short-eared  rabbits.  It  is  a  curve  convex  above,  indicating 
a  steadily  diminishing  daily  increment  in  ear-lengtli. 


GROWTH-RATE  OF  LOP-EARED  AND  OF   SHORT-EARED  RABBITS  IN  SIZE 

AND  IN   EAR-LENGTH. 

The  theoretical  growth-curve  of  an  organism  in  weight  (Houssay,  loy; 
Robertson,  :o8)  is  at  first  concave  upward,  but  later  becomes  convex. 
When  the  curve  is  concave  upward  the  daily  growth  increment  is  increas- 
ing. But  when  the  growth-curve  becomes  convex  upward,  it  is  evident 
that  the  growth  increment  is  decreasing.  Therefore  the  period  of  greatest 
daily  growth  occurs  when  the  growth-curve  is  changing  from  a  concave 
to  a  convex  one.  In  rabbits  this  occurs  at  an  age  of  from  6  to  8  weeks 
after  birth  (see  figs,  i  to  3).  According  to  Robertson  (:o8)  the  period  of 
maximum  growth  corresponds  with  the  middle  point  of  a  growth-cycle 
which  in  character  resembles  an  autocatalytic  monomolecular  chemical 
reaction.  In  the  rabbit  this  growth-cycle  probably  has  its  beginning  at 
some  time  prior  to  birth  and  ends  before  puberty  is  attained. 

It  is  possible  that  this  same  form  of  curve  would  be  observed  in  respect 
to  ear-length  also,  if  the  measurements  began  at  a  period  sufficiently  early. 
Growth  of  the  ears  is  completed  before  increase  in  body-weight  ceases, 
and  it  is  possible  that  the  growth-curve  for  ear-length  has  already  changed 
from  a  concave  to  a  convex  form  at  the  age  of  2  weeks,  when  our  measure- 
ments begin.  But  it  is,  on  the  other  hand,  possible  that  the  growth-curve 
for  ear-length  would  not  show  a  convex  form  upward  even  if  completed 
for  the  period  prior  to  2  weeks  of  age;  for  ear-length  is  a  linear  dimen- 
sion, whereas  body- weight  depends  on  volume,  i.  e.,  size  in  three  dimen- 
sions, and  a  doubHng  of  any  linear  dimension  should  be  attended  by  an 
eight-fold  increase  of  volume. 

A  comparison  of  fig.  i  with  fig.  2  shows  a  considerable  difference  between 
ordinary  short-eared  (fig.  i)  and  lop-eared  (fig.  2)  rabbits  as  regards  size, 
at  corresponding  ages;  the  difference  is  even  more  striking  in  regard  to 
ear-length.  Crosses  between  the  two  varieties  produce  rabbits  inter- 
mediate in  character  as  regards  both  weight  and  ear-length.  But  before 
considering  further  the  character  of  the  cross-breds,  it  will  be  well  to  inquire 
how  each  variety  breeds  by  itself. 


'  The  measurements  for  ear- width  and  "spread"  are  closely  correlated  with  those  for  ear-length. 
For  the  sake  of  simplicity  we  shall  deal  with  the  statistics  for  ear-length  only. 


EAR-SIZE 


13 


Ear  length 
in  mm. 


220- 


210 


200- 


190 


130 


no- 


lea- 


150- 


140 


130 


120 


110 


100- 


90 


80 


70 


60 


SO- 


40 


30 


ti 

J 

^ 

4 

cf 
i 

¥ 

i 
1 
i 

i     ' 

* 

i       > 

4t 

•          JO 

i 

1      cr  .■ 

7  ?'      f 

Aqe  in       '     ' — ' — ■ — ' — i — ' — ' — r— i 1 1 — r 1 — i — \ 1 1 1 — r- 

v/eeks.      '    2    3  4    5    6    7    6   9  10  II  12  13  14  I5  16  17  18  13  20 

Fig.  2.   Growth-curves  for  a  litter  of  five  lop-eared  rabbits. 
See  table  2  and  compare  fig.  i. 


14 


INHERITANCE    IN   RABBITS 


MATINGS   OF   SHORT-EARED   RABBITS   ENTER   SE. 

Several  matings  of  short-eared  rabbits  inter  se  are  recorded  in  table  i. 
They  show  great  uniformity  of  result.  The  young  differ  little  in  ear- 
length  from  their  parents,  which  in  no  case  differed  from  each  other  by 
more  than  5  mm. 

Table  i. 


Mating  i. 

Mating  2. 

Ear- 
length. 

Weight. 

Age. 

Ear- 
length. 

Weight. 

Age. 

Parents : 

9  255 

d'497 

Mid-parental 
Offspring: 
Litter  i  — 

9768 

9  769 

9770 

c?77i 

9772 

c?773 

Litter  2  — 

c?88i 

?882 

c?883 

d>884 

mm. 

"5 
no 

112.5 

107 
in 
"5 
"3 
106 
III 

no 
no 
114 
"5 

gms. 

2,650 
2,04s 

2,347 

1,445 
1,480 
1,780 
1,740 
1,550 
1,650 

1,380 
1,380 
1,500 
1,560 

Adult.» 
7  mos. 

20  weeks. 
Do. 
Do. 
Do. 
Do. 
Do. 

14  weeks. 
16  weeks. 
14  weeks. 
Do. 

Parents : 

9498 

(^497 

Mid-parental 
Offspring : 
Litter  i  — 

9782 

9783 

9784 

,cf785-- 

Litter  2  (see 
fig.  I)  - 

9840 

9841 

9842 

c?843 

9844 

9845 

Tntn, 

no 
no 
no 

no 
106 
107 
no 

112 
1x6 
106 
in 

"5 
no 

gms. 

1,980 
(  1.91S 
\  2.045 

1.947 

1.365 
1.575 
1.375 
1.695 

1,450 
1,460 
1,460 
1,510 
1,340 
1,420 

27weeks. 

7  mos. 
27  weeks. 

20  weeks. 
Do. 
Do. 
Do. 

18  weeks. 
Do. 
Do. 
Do. 
Do. 
Do. 

Mating  3. 

Mating  4. 

Parents: 

$268 

(5^497 

Mid-parental 
Offspring : 

9859 

c?86i 

d>862 

c?863 

105 
no 

107-5 

no 
112 
no 
no 

2,280 

2,045 
2,162 

1,445 
1,340 
1,420 

1,295 

Adult. 
7  mos. 

15  weeks. 
Do. 
Do. 
Do. 

Parents: 

9268 

c?  56 

Mid-parental 
Offspring: 

0^774 

105 
102 

103-5 
108 

2,280 
2,500 
2,390 

1,715 

Adult. 
Do. 
Do. 

20  weeks. 

1  By  adult  is  meant  i  year  or  more  old. 


pTn  mating  i ,  the  extreme  deviations  from  the  mid-parental  *  ear-length 
are  —6.5  mm.  and  +2.5  mm.,  the  average  deviation  being  only  2.5  mm. 
The  total  range  of  variation  is  9  mm.  In  mating  2,  between  brother  and 
sister,  the  extreme  deviations  are  —4  mm.  and  -f-  6  mm.,  giving  a  total 
range  of  variation  of  10  mm.  The  average  deviation  from  the  parental 
ear-length  (no  mm.)  is,  as  in  mating  i,  2.5  mm, 

1  By  mid-parental,  as  we  shall  use  the  term  in  this  paper,  is  meant  a  magnitude  exactly  halfway 
between  the  magnitudes  of  the  respective  parents.     It  is  the  mean  of  the  parental  magnitudes. 


EAR-SIZE 


15 


The  growth-curves  for  litter  2,  which  were  produced  by  this  mating, 
are  shown  in  figure  i.  In  mating  3,  the  deviations  are  all  plus  in 
character,  but  are  small  in  amount,  namely,  2.5,  4.5,  2.5,  and  2.5  mm. 


Ear  length 

IT)  mm. 
160- 


no 


160- 


150- 


140 


130 


120 


no 


100 


90- 


BO 


10 


60 


SO 


40- 


30- 


Age  m 
weeks. 


^ 


■(/ 


^789 

p. 

ci   f788 

-0  V  / 

r 

t 

j>-o  787 

—r—r—i 1 1 J        I        I        I        I        J 1 1 1       I '      I        I        I        I        I        I 

/    2    3    4    5    6    7    8    9   10  n  12  13  14  15  16  17  18  19  20  2/ 


Fig.  3.     Growth-curves  for  a  litter  of  five  second-generation  (F,^)  half-lop  rabbits. 
See  table  9  and  compare  figs.  I  and  2. 


From  mating  4,  by  the  same  female  that  was  concerned  in  mating  3,  a 
single  young  one  was  reared,  which  showed  a  plus  deviation  of  4.5  mm. 


16 


INHERITANCE    IN   RABBITS 


Another  mating  which  falls  in  this  category  was  made  between  the 
Belgian  hare  (9  431)  and  the  short-eared  c?  56  (see  table  ia).  It  shows 
a  complete  blending  in  the  offspring  of  the  parental  ear-lengths,  with  a 
very  small  range  of  variation,  viz,  6  mm. 


Table  ia. 


Ear- 
length. 

Weight. 

Age. 

Parents : 

9431 

r?>   1:6 

mm. 

n8 
102 
no 

I  102 

I  105 

III 

108 

J  108 

1  no 

108 

gms. 

3.400 
2,500 
2.95° 

2,700 
2,945 

Adult. 
Do. 
Do. 

21  weeks. 
Adult. 
21  weeks. 

Do. 

Do. 
Adult. 
21  weeks. 

Mid-parental .... 

Offspring : 

9232 

Q  2X1     

6^234 

c?23S 

9236 

The  mid-parental  ear-length  was  exceeded  by  i  of  the  young  at  21 
weeks  of  age;  3  others  came  within  2  mm.  of  the  mid-parental  ear-length 
at  21  weeks  of  age,  and  i  of  these  equaled  it  when  adult.  If  the  other  2 
did  as  well  they  too  must  have  attained  the  expected  ear-length.  Only 
I  individual  (9232),  then,  fails  to  attain  the  mid-parental  ear-length. 
This  result  is  almost  identical  in  general  character  with  that  shown  by 
table  I. 

We  may  conclude  that  short-eared  rabbits  breed  true  within  a  range  of 
fluctuating  variability  not  exceeding  10  mm. 


MATINGS   OF   LOP-EARED   RABBITS   ENTER   SE. 

Our  original  stock  of  lop-eared  rabbits  consisted  of  a  single  pair.  Both 
of  them  gave  vigorous  young  in  matings  with  short-eared  rabbits,  but  not 
with  each  other.  Consanguinity  may  have  been  the  reason  for  this  lat- 
ter fact.  They  were  obtained  from  the  same  source,  and  doubtless  were 
nearly  related,  as  well  as  inbred.  Nevertheless  we  did  obtain  from  them 
two  good-sized  and  healthy  young,  c?  179  and  9  180.  The  former  appears 
in  many  of  the  crosses  to  be  described,  but  the  latter  proved  a  very  poor 
mother,  producing  only  occasional  htters  of  young,  none  of  which  attained 
maturity.  Table  2  shows  the  only  results  obtained  from  mating  lop-eared 
individuals  inter  se. 

Mating  i  produced  2  young,  one  (^  179)  very  similar  to  the  father,  the 
other  (9  180)  very  similar  to  the  mother,  but  not  quite  so  large  and  with 
ears  5  mm.  shorter.  The  deviations  from  the  mid-parental  ear-length 
are  —7.5  and  —2.5  mm.,  respectively. 


EAR-SIZE 


17 


Mating  2  (between  brother  and  sister)  produced  2  young,  which  reached 
the  age  of  20  weeks.  Though  they  were  not  large,  their  ears  attained 
a  good  length,  the  deviations  from  the  mid-parental  ear- length  being 
—  5  mm.  and  —2  mm. 

Table  2. 


Mating  i. 

Mating  3. 

Ear- 
length. 

Weight. 

Age. 

Ear- 
length. 

Weight. 

Age. 

Parents : 

Old  9  lop  (pi. 

I,  fig.  2). . 

Old  c?  lop   .  . 

Mid-parental 

Ofifspring: 

c?i79(pl.  2,fig. 

8) 

9 180 

mm. 

225 
210 

217-5 

210 
220 

gms. 

4,600 

3.450' 
4,025 

3.410 
3.765 

Adult. 
Do. 
Do. 

Adult. 
Do. 

Parents: 

Old  9  lop  (pi. 

I,  fig-  2)    . 

0^179   (Pl-    2, 

fig- 8) 

Mid-parental 

Ofifspring   (see 
fig.  2): 

cf667 

9668 

9669 

c?670 

(^671 

mm. 

225 

210 
215-7 

1  320 
1  223 

300 

215 

330 
190 

gms. 

4,600 

3.410 
4,005 

2,010 

1. 590* 

«,37o 

2,030 

1,900 

1.205 

Adult. 

Do. 
Do. 

14  weeks. 
20  weeks. 
14  weeks. 

Do. 

Do. 

Do. 

Mating  2. 

Parents: 

9  180 

<?i79 

Mid-parental 
Offspring: 

0^616 

6^618 

210 
220 
215 

210 
213 

3.410 
3.765 
3,587 

1,680 
1,820 

Adult.      ' 
Do. 
Do. 

20  weeks. 
Do. 

1  Estimated. 


'Sick. 


Mating  3  (between  mother  and  son)  produced  a  litter  of  5  young,  which 
grew  in  a  satisfactory  manner  until  14  weeks  old  (see  fig.  2).  Then, 
as  a  result  of  disease,  4  of  them  died,  and  the  fifth  became  greatly  reduced 
in  flesh,  so  that  at  20  weeks  of  age  he  weighed  400  grams  less  than  at 
14  weeks  of  age.  Nevertheless  his  ears  continued  to  grow  slowly.  At  14 
weeks  they  measured  220  mm.;  at  20  weeks,  223  mm. 

The  rabbit  671  was  from  the  beginning  much  the  smallest  one  in  the 
litter;  we  named  him  the  "runt"  and  had  hopes  of  securing  from  him  a 
race  of  small-sized  but  lop-eared  rabbits.  These  hopes  were  ended  by 
the  unfortunate  illness  which  attacked  the  entire  litter.  The  small  size 
of  this  rabbit  accounts  for  the  shortness  of  his  ears  (190  mm.  at  14  weeks 
of  age).  Leaving  him  out  of  consideration,  the  range  of  variation  in  ear- 
length  is  20  mm.;  with  him,  it  is  30  mm.,  at  14  weeks  of  age. 

Two  of  the  young  produced  by  mating  3  had  already  at  14  weeks  of 
age  exceeded  the  mid-parental  ear-length,  a  third  had  almost  reached 
it,  while  the  2  others  fell  below  it.  This  is  a  fluctuating  variation  around 
the  mid-parental  ear-length,  and  indicates  that  the  long-cared  character 
tends  to  breed  true,  within  a  range  of  variation  of  20  (or  possibly  30)  mm., 
the  minus  variations,  however,  probably  being  greater  than  the  plus  ones. 


18 


INHERITANCE  IN  RABBITS 


Cross  i.  —  Lop-eared  Female  X  Short- eared  Male. 

The  largest  and  longest-eared  rabbit  with  which  we  have  experimented 
was  a  female  obtained  by  purchase  and  of  unknown  ancestry.  (See  plate  i, 
fig.  2.)  Her  ear-length  was  225  mm.  and  her  adult  weight  4,600  grams. 
She  was  mated  with  a  small-eared  angora  rabbit  (c?  45,  plate  i,  fig.  3), 
whose  ear-length  was  105  mm.  and  adult  weight  3,000  grams.  A  htter 
of  8  young  was  obtained  from  this  pair.  All  were  reared  to  an  age  of 
2  months,  when  6  were  discarded.  The  remaining  2  were  reared  to  matu- 
rity. One  of  them  (<?248)  is  shown  in  plate  i,  fig.  i.  The  6  discarded 
rabbits  had  ears  shorter  than  those  of  the  rabbits  which  were  kept.  Their 
ear-lengths  are  given  in  table  3  as  estimated  from  the  known  relation  of 
their  ear-lengths  at  2  months  of  age  to  the  ear-lengths  of  rabbits  247  and 
248,  the  animals  kept  until  adult.  Table  3  shows  that  the  young  obtained 
from  this  cross  are,  as  regards  ear-length,  intermediate  between  the  parents, 
but  stand  nearer  the  short-eared  than  the  long-eared  parent.  As  regards 
weight,  9  247  is  smaller  and  c?  248  larger  than  the  mid-parental  condition; 
the  remaining  6  would  probably  not  have  exceeded  $  247  in  weight  had 
they  been  reared  to  maturity.  Accordingly  as  regards  both  size  and  ear- 
length  in  this  cross  the  resemblance  is  greater  toward  the  smaller  and 
shorter-eared  parent  (father). 

Table  3.  —  Cross  i. 


Ear- 
length. 

Weight. 

Age. 

Parents: 

9  lop     

tnvi . 

225 
105 
i6s 

152 
153 
145^ 
147  » 
138  1 
149  1 

145' 
142  1 

gms. 

4,600 
3,000 
3,800 

3.290 
3.930 

Adult. 
Do. 
Do. 

Adult. 
Do. 

r?  0.1; 

Mid-parental.  .  .  . 
Offspring: 

Q  247 

c?248 

C^2A0 

rT'a^o 

C?2Si 

(3^252 

c?253 

c?254 

»  Estimated. 


Cross  2.  —  Short-eared  Female  x  Lop-eared  Male. 

This  cross,  the  reciprocal  to  the  foregoing,  was  made  repeatedly.  The 
lop-eared  male  used  {d  179,  plate  2,  fig.  8)  was  a  son  of  the  lop-eared 
female  employed  in  cross  i.  He  was,  how^ever,  smaller  than  his  mother, 
and  had  shorter  ears.  The  results  of  4  different  matings  are  shown  in 
table  4.  In  mating  i  there  were  only  2  surviving  young,  which  there- 
fore were  probably  the  largest  and  strongest  individuals  in  the  Utter  and 
received  more  than  the  average  amount  of  nourishment.     One  of  them 


EAR-SIZE 


19 


surpassed  at  i8  weeks  of  age  the  mid-parental  car-length,  while  the  other 
one  almost  equaled  it.  Their  weights  at  i8  weeks  of  age  indicated  that 
the  mid-parental  weight  would  be  attained  at  maturity. 


Table  4.  —  Cross  2. 


Mating  i. 

Mating  3. 

Ear- 
length. 

Weight. 

Age. 

Ear- 
length. 

Weight. 

Age. 

Parents: 

9  268 

c?i79lop  . .  . 
Mid-parental 
Offspring: 

c?57i 

9572 

mm. 

105 
210 

157-5 

155 
160 

gms. 

2,280 

3.410 
2.84s 

2,310 
2,1 10 

Adult. 
Do. 
Do. 

18  weeks. 
Do. 

Parents: 

9ios 

c?i79(lop) 

Mid- parental 
Offspring: 

9626 

(^627 

9639 

9630 

9633 

(5*634 

mm. 

TOO 
310 

155 

ISO 
ISO 
150 
I4S 
148 

ISO 

gms. 

3.410 

1.380 
1.470 
1.485 
1.430 
1.370 
1.32s 

9  mos. 

Adult. 

30  weeks. 
Do. 
Do. 
Do. 
Do. 
Do. 

Mating  2. 

Mating  4. 

Parents: 

9269  (pi.  2, 

fig-  5)  '  •  '  ■ 
c?!  79  (lop).. 
Mid-parental 
Offspring: 
Litter  i  — 

9574 

,  9575 

Litter  2  (pi.  2, 
fig-  6)  - 

c?640 

9641 

9  642 

9643 

92 
210 
151 

140 
143 

150 
150 
152 
144 

1.935 
3.410 
2,672 

1.950 
1,940 

1,980 
1.850 
1.930 
1,670 

Adult. 
Do. 
Do. 

18  weeks. 
Do. 

20  weeks. 
Do. 
Do. 

19  weeks, 
(less  at  20) 

Parents: 

9 108 

c?i79 

Mid-parental 
Offspring: 

0^607 

6^608 

c?6o9 

no 

3IO 
160 

158 

iSS 

158 

2,520 
3.410 
2,965 

2,050 
1,810 
3,080 

Adult. 
Do. 
Do. 

20  weeks. 
18  weeks. 
Do. 

Under  the  head  of  mating  2  arc  given  the  results  of  2  dilTerent  litters, 
litter  I  consisting  of  2  rabbits,  litter  2  of  4.  The  weight  of  the  mother 
was  surpassed  by  that  of  the  ofTspring  in  4  out  of  6  cases,  at  the  early  age 
of  18  to  20  weeks.  Three  of  the  6  young  had  ear-lengths  very  similar  to  the 
mid-parental  car-length;  the  remaining  3  had  ears  somewhat  shorter  at 
18  or  19  weeks  old,  and  probably  would  not  have  attained  at  maturity 
an  ear-length  equal  to  the  mid-parental.  Litter  2  is  shown  in  plate  2, 
fig.  6;  the  parents  in  figs.  5  and  8  of  the  same  ])late. 

The  6  young  produced  by  mating  3  were  under-sized  at  20  weeks  of 
age,  which  perhaps  accounts  for  the  fact  that  no  one  of  them  attained  the 
mid-parental  ear-length,  but  all  fell  from  5  to  10  mm.  short  of  it. 

In  mating  4,  2  of  the  3  young  came  within  2  mm.  of  attaining  the  mid- 
parental  ear-length;  the  third  came  within  5  mm.  of  it. 


20 


INHERITANCE  IN   RABBITS 


On  the  whole,  the  result  of  cross  2  is  a  fairly  close  approximation  to 
the  mid-parental  ear-length.  In  no  case  does  the  deviation  from  the  mid- 
parental  value  exceed  11  mm.;  the  average  deviation  from  it  is  only  4.8 
mm.  But  the  differences  between  the  respective  parents  ranged  from  100 
to  118  mm.,  and  the  least  deviation  of  one  of  the  offspring  from  either 
parent  was  45  mm.  or  more  than  four  times  the  greatest  deviation  from 
the  mid-parental  value.  When  deviation  from  the  mid-parental  value 
did  occur,  it  was  oftener  under  than  over  the  mid-parental  value. 

Accordingly,  the  results  observed  as  regards  ear-length  may  accurately 
be  described  as  a  blend.  As  regards  body-size,  the  data  are  insufficient, 
since  adult  weights  of  the  offspring  were  in  no  case  obtained,  but  the 
observed  weights  of  the  offspring  in  matings  i  and  2  indicate  that  a  blend 
might  be  expected,  an  intermediate  condition  having  already  been  obtained 
at  20  weeks  of  age. 

Cross  3.  —  Belgian  Hare  Female  X  Lop-eared  Male. 

The  "Belgian  hare"  (plate  3,  fig.  9)  used  in  this  cross  was  larger  and 
had  somewhat  longer  ears  than  the  short-eared  rabbits  used  in  crosses  i 
and  2.  The  lop-eared  male  was  father  of  the  one  used  in  cross  2,  but  had 
about  the  same  ear-length  and  body-size.  A  Htter  of  6  young  was  ob- 
tained, five  of  which  were  reared  to  an  age  of  21  weeks  or  more.  In  size 
the  offspring  exceeded  either  parent,  approximating  that  of  the  female 
lop  used  in  cross  i.  Four  of  the  5  young  also  exceeded  the  mid-parental 
ear-length  by  from  2  to  6  mm.,  but  the  fifth  fell  short  of  it  by  8  mm.  This 
same  individual  (diyj)  showed  the  least  deviation  from  either  parental 
ear-length,  viz,  38  mm.,  or  four  and  a  half  times  the  greatest  deviation 
from  the  mid-parental  ear-length. 


Table 

5.       Cross  3. 

Ear- 
length. 

Weight. 

Age. 

Parents: 

9  43 1  (Belgian  hare) .  . 
Old  c?  lop 

mm. 

118 
210 
164 

170 
170 
166 

156 

170 

gms. 

3,400 

3.4So(?) 

3,425(?) 

4,305 
4,130 

4,070 

Adult. 
Do. 
Do. 

21  weeks. 
Adult. 
Do. 

21  weeks. 
Adult. 

Mid-parental 

Offspring: 

r^ITA. 

QI7I: 

(5^176 

d'177 

$178 

Accordingly,  in  cross  3,  as  in  cross  2,  the  ear-length  of  the  offspring  is 
approximately  a  blend  of  the  ear-lengths  of  the  respective  parents.  The 
size  of  the  offspring,  however,  is  greater  than  that  of  either  parent,  though 
it  does  not  exceed  the  size  of  lop-eared  individuals  other  than  the  father. 


EAR-SIZE 


21 


Cross  4.  —  Lop-eared  Female  x  Half-blood  Lop  Male. 

This  cross  is  a  sequel  to  crosses  i  and  3,  a  male  rabbit  j^roduced  by 
cross  3  being  mated  with  the  female  lo])  used  in  cross  i.  This  cross  pro- 
duced three-quarter-blood  lops,  the  ear-lengths  of  which  are  indicated  in 
table  6. 

Table  6.  —  Cross  4. 


Ear- 
length. 

Weight. 

Age. 

Parents: 

Old  9  lop 

mm. 

225 
166 

'955 

186 
206 
192 
180 

(  185 
(  210 

1  190' 
/  200 

182 

183 

19s 

gms. 

4,600 
4.130 
4.365 

3.430 
3.770 
3.150 
1,690 

3.910 

3.465 
3.955 



4.450 

Adult. 
Do. 
Do. 

I  year. 
42  weeks. 
30  weeks. 
18  weeks. 
16  weeks. 

I  year. 

25  weeks. 

I  year. 
16  weeks. 
18  weeks. 

I  year. 

J'176 

Mid-parental 

Offspring: 
Litter  i  — 

Q  eo4 

(-?coe 

cJ'506 

9  C08 

Q  coo 

Litter  2  — 

r?'7io       

9320 

9321 

9322  (pi.  3,  fig.    11).  . 

'  23  weeks. 

The  range  of  variation  in  this  mating  is  similar  to  that  observed  in  a 
mating  of  this  same  female  with  a  lop-eared  male  (see  table  2),  viz,  a 
variation  of  between  20  and  30  mm.  It  is  difficult  to  estimate  it  more 
precisely,  because  the  measurements  recorded  were  made  at  such  differ- 
ent ages.  Two  of  the  offspring  apj>ro.\imate  the  mid-parental  conditions 
both  of  ear-length  and  of  weight,  these  two  being  (^319  and  9322.  The 
same  is  measurably  true  of  a  third  individual,  s  506.  But  9  504  and 
9  508  fall  considerably  short  of  the  mid-parental  ear-length  and  the 
mid-parental  weight;  while  c?  505  and  9  509  con.siderably  exceed  the  mid- 
parental  ear-length,  and  approximate  the  mid-parental  size.  The  greatest 
deviation  from  the  mid-parental  ear-length  is  a  minus  one  of  15  mm. 
(recorded  at  the  age  of  18  weeks),  but  a  \Aus  deviation  of  nearly  the  same 
amount  (14.5  mm.)  is  also  observed,  though  not  until  the  age  of  a  year 
had  been  attained.  The  average  deviation  from  mid-parental  ear-length 
is  9  mm.  The  lowest  measurement,  180  mm.,  stands  almost  exactly 
midway  between  the  ear-length  of  the  father  (166  mm.)  and  the  mid- 
parental  ear-length  195.5  mm.;  while  the  largest  measurement  (210  mm.) 
stands  midway  between  the  condition  of  the  mother  (225  mm.)  and  the 
mid-parental  ear-length. 

The  result  observed  in  this  cross  may  be  de.scribed  as  blending  inheri- 
tance, with  fluctuation  about  the  mid-parental  ear-length,  in  about  the 
same  degree  as  in  the  case  of  lop-eared  rabbits  i)urely  bred. 


22 


INHERITANCE   IN  RABBITS 


Two  of  the  offspring  produced  by  cross  4  were  mated  with  each  other, 
viz,  9  504  and  c?  506.  They  produced  a  htter  of  3  young,  the  character 
of  which  is  shown  in  table  7. 

Table  7. 


Ear- 
length. 

Weight. 

Age. 

Parents: 

9  <;o4 

mm. 

186 
192 
189 

191 
201 
185 

gms. 

3.430 
3.150 
3.290 

2,255 
2,255 
1,900 

I  year. 

30  weeks. 

Do. 

20  weeks. 

Do. 
19  weeks. 

f^irod 

Mid-parental .... 
Offspring: 

rt^TAO 

r?'?^.! 

$742 

The  deviations  from  the  mid-parental  ear-length  are  +2,  —12,  and 
—  4  mm.  respectively,  which  lie  within  the  limits  of  variation  observed 
among  lop-eared  rabbits  purely  bred. 

The  range  of  variation  is  chiefly  upward  (plus),  doubtless  because  of 
the  small  number  in  the  htter,  which  would  make  the  food  supply  of  the 
individual  better  than  usual. 


Cross  5.  —  Half-blood  Lop  Female  x  Lop  Male  179. 

Female  167  was  produced  by  a  cross  similar  to  cross  2,  in  which  the 
mother  was  a  short-eared  Himalayan  rabbit  (9  23),  and  the  father  the 
male  lop  used  in  cross  3.  She  was  mated  with  lop  c?  179  and  produced 
3  htters  of  young,  as  indicated  in  table  8. 

The  range  of  variation  in  ear-length  in  these  3  htters  of  rabbits  is  wide, 
extending  from  165  mm.  in  a  rabbit  7  months  old  (having  full-grown 
ears)  to  194  mm.  in  one  only  18  weeks  old,  or  to  200  mm.  in  one  30  weeks 
old,  a  total  range  of  35  mm.  The  largest  minus  deviation  from  the  mid- 
parental  ear-length  is  5  mm. ;  the  largest  plus  deviation,  30  mm. ;  the  least 
deviation  from  the  short-eared  parent  is  35  mm.,  but  from  the  long-eared 
parent  only  10  mm.  Hence  in  this  cross  the  long-eared  parent  is  approx- 
imated more  closely  than  the  short-eared  one.  This,  however,  is  not  of 
necessity  evidence  of  Mendehan  recessiveness  of  the  character  long-ear 
in  9  167.  Another  and,  we  believe,  better  way  of  viewing  the  matter  is 
this:  9  167  transmitted  a  greater  ear-length  than  she  had  herself  attained. 
It  is  known  that  conditions  which  influence  general  growth,  during  the  first 
20  weeks,  influence  also  ear-length.  But  at  20  weeks  of  age  ear-growth 
is  practically  complete,  although  growth  in  other  respects  continues  some 
time  longer.     It  is  possible,  therefore,  for  an  animal  to  be  stunted  in  ear- 


EAR -SIZE 


23 


size  and  yet  lo  attain  a  normal  or  nearly  normal  general  size.  It  is  not 
to  be  expected,  however,  that  such  an  animal  will  transmit  to  voung  reared 
under  normal  conditions  the  diminished  ear-size  which  it  shows,  but  rather 
the  ear-size  which  it  would  have  attained  harl  it  been  reared  under  normal 
conditions. 

Table  8.  —  Cross  5. 


Parents: 

?i67 

cJ>i79(lop)    . 
Mid-parental 

Offspring: 
Litter  i  — 
c^437 

9438 

Litter  2  — 

^^566 

crs68 

(^'sfig 

9570 

Litter  3  — 
9644 

(^645 

9646 

9647 

9648 

cr649 


Ear- 
length. 


mm. 

130 
210 
170 


1  184 
/  200 

I  177 
/  185 


Weight. 


gms. 

2,55° 
3.410 
2,980 


2.550 
3.140 
3,510 
3. no 


194 

1.945 

181 

1.915 

170 

1,865 

183 

1 164 

1.655 

1  i6s 

3,430 

170 

1,460 

175 

1,430 

171 

2,030 

180 

1.930 

178 

2,080 

Age. 


Adult. 
Do. 
Do. 


20  weeks. 
30  weeks. 
20  weeks. 
30  weeks. 

18  weeks. 
Do. 
Do. 
Do. 

20  weeks. 

7  months. 
20  weeks. 

Do. 

Do. 

Do. 

Do. 


Cross  6.  —  Half-blood  Lops  Mated  inter  se. 

The  same  female  half-blood  lop  already  mentioned  (9167,  cross  8), 
was  mated  with  a  male  produced  by  cross  i  (^"248).  Their  young  con- 
stitute an  F2  generation  of  half-blood  lops. 

In  this  litter  the  deviations  from  the  mid-parental  ear-length  are  all, 
with  one  exception,  positive  (upward).  This  result  accords  with  that 
observed  among  the  young  of  this  same  mother  (9  167),  in  connection 
with  cross  5.  She  evidently  transmitted  a  greater  car-length  than  she 
manifested. 

The  range  of  variation,  35  mm.,  while  high,  docs  not  exceed  that  found 
among  lop-eared  rabbits  mated  inter  se,  as  is  clear  from  a  comparison  of 
fig.  2  with  fig.  3,  the  former  showing  growth-curves  for  lop-eared  rabbits, 
the  latter  for  the  litter  of  F,  half-blood  rabbits  under  consideration.  The 
range  of  variation  in  this  cro.ss  also  agrees  exactly  with  that  observed  in 
cross  5,  in  which  the  same  mother  was  matccl  with  a  full-blood  lop.  We 
get,  therefore,  from  this  case  no  evidence  of  Mendelian  sj)litting  as  regards 
the  character  ear-length. 


24 


INHERITANCE   IN   RABBITS 


Table  9. 


Ear- 
length. 

Weight. 

Age. 

Parents: 

Q167      

mitt. 

130 
153 
141-5 

140 

153 
170 

175 
140 

gms. 

2,550 
3.930 
3.240 

1.730 

1.915 
2,060 

2,170 

2,155 

Adult. 
Do. 
Do. 

20  weeks. 
Do. 
Do. 
Do. 
Do. 

c?248 

Mid-parental.  . . . 
Offspring : 

c?786 

d^787 

c?788 

Q  780 

(^792 

Two  of  the  young  produced  by  cross  5  were  mated  with  each  other, 
viz,  c?437  with  9438.  Their  young  (table  10)  vary  closely  about  the 
mid-parental  ear-length. 

Table  10. 


Ear- 
length. 

Weight. 

Age. 

Parents: 

$4.78 

mm. 

184 
177 
180.5 

180 

185 
180 
176 

gms. 

2,550 
2,510 
2,530 

1,970 
2,030 
2,010 
1,825 

20  weeks. 
Do. 

20  weeks. 
Do. 
Do. 
Do. 

(^Aiy 

Mid-parental  .... 
Offspring : 

r^yiQ 

$720 

(^721 

r?*725 

Another  cross  6  mating  was  obtained  between  9  247  and  d  248  (pro- 
duced in  cross  i).     The  character  of  the  young  is  shown  in  table  11. 


Table  ii. 

Ear- 
length. 

Weight. 

Age. 

Parents : 

Q247 

mm. 

152 
153 
152-5 

122 
130 
135 
135 
130 

gms. 

3,290 
3,930 
3,610 

1,375 
1,770 
2,460 
1,860 
1,755 

Adult. 
Do. 
Do. 

19  weeks. 

Do. 
8  months. 
19  weeks. 

Do. 

c?248 

Mid-parental  .... 
Offspring : 

Q  -loy 

(f-iaS 

Q  400 

Q401      

(^402 

Contrary  to  the  result  shown  in  table  9,  the  young  obtained  from  this 
mating  all  fall  short  of  the  mid-parental  ear-length  by  from  17  to  29.5 
mm.,  indicating  probably  conditions  of  nutrition  below  the  normal,  dur- 
ing the  period  of  principal  growth  of  the  ears,  or  of  the  transmission  by 
the  parents  of  a  condition  of  ear-length  inferior  to  that  which  they  mani- 


c. 


EAR-SIZE 


25 


fested.  The  young  vary  in  normal  fashion  about  a  mean  ear-length  of 
130.4  mm.  The  total  range  i.s  only  13  mm.,  indicating  no  Mendclian 
heterogeneity  among  the  gametes  [produced  by  the  jjarents,  though  both 
were  Fj  half-lops. 

One  of  the  young  produced  in  this  Utter  (9400)  was  mated  with  the 
lop-eared  cfiyg,  table  2.  The  result  is  shown  in  table  12.  Six  offspring 
were  obtained  from  this  mating;  they  vary  rather  closely  about  the  mid- 
parental  ear-length,  though  chiefly  below  it,  as  we  might  e.xpect  from  the 
fact  that  the  mid-parental  value  given  is  based  upon  measurements  of 
adults,  and  that  of  the  young  upon  measurements  at  the  age  of  20  weeks. 
The  total  range  of  variation  is  15  mm. 


Table  12. 

Ear- 
length. 

Weight. 

Age. 

Parents: 

Q  AOO 

mm. 
135 

310 
172.5 

162 
166 
160 
171 
160 
175 

gms. 

2,460 
3.410 
2.935 

1,680 
1,670 
1,520 
1,880 

1.715 
1,720 

8  months. 
Adult. 

20  weeks. 
Do. 
Do. 
Do. 
Do. 
Do. 

r^iTO 

Mid-parental. . . . 
Offspring : 

c^'JOX 

C^noA 

2  70c 

r?7o6 

$  707 

Q  708 

Female  400  was  hkewise  mated  with  her  father  (d"248),  producing  a 
litter  of  4  young,  all  of  which  fell  below  the  mid-parental  ear-lengths. 
(See  table  13.) 

Table  13. 


Ear- 
length. 

Weight. 

Age. 

Parents : 

Q  AOO 

mm. 

135 
153 
144 

135 
137 
138 
126 

gms. 

2,960 
2,930 
3.445 

1,500 
1.710 
1.780 
1,560 

8  months. 
Adult. 

20  weeks. 
Do. 
Do. 
Do. 

f?248     

Mid-parental .... 
Offspring : 

9818 

C^SlQ 

9820 

9821 

The  deviations  are  —9,  —7,  —16,  and  —18,  average  —12.5  mm.  But 
the  total  range  of  variation  is  only  11  mm.,  or  scarcely  greater  than  that 
observed  among  short-eared  rabbits.  Certainly  tliis  result  affords  no 
evidence  of  heterogeneity  as  regards  ear-character  among  the  gametes 
formed  by  the  parents,  though  one  was  an  Fj  and  the  other  an  Fj  cross- 
bred between  the  lop-eared  and  the  short-eared  races. 


26 


INHERITANCE   IN   RABBITS 


OTHER  MATINGS  OF  HALF-BLOOD  LOPS  AND  ADDITIONAL  CROSS  6  MATINGS. 

Other  matings  in  which  the  rabbits  9  247  and  c?  248  were  concerned 
are  recorded  in  tables  14  and  15. 

In  mating  i  (with  d  179)  9247  gave  a  fully  normal  blending  result. 
Of  the  5  young  produced,  2  siu-passed  the  mid-parental  ear-length,  i 
equaled  it,  and  2  fell  below  it.  All  were  intermediate,  and  the  range  of 
variation  was  14  mm.,  or  about  one-fourth  of  the  difference  between  the 
parents.  In  mating  2  (with  c?3i9)  9247  gave  a  result  similar  to  that 
which  she  had  given  with  c?  248.  All  the  young  were  intermediate  in 
ear-length,  but  all  fell  short  of  the  mid-parental  ear-length,  by  from  3  to 
16  mm.  This  was  not  due  to  consanguinity,  for  9247  and  (^319  were 
not  closely  related.  It  may,  however,  have  been  due  to  inferior  condi- 
tions of  nutrition,  perhaps  resulting  from  the  large  size  of  the  litter.  The 
whole  litter  seems  to  have  been  affected  ahke,  the  total  range  of  variation 
among  the  seven  young  being  only  11  mm. 

Table  14. 


Mating  i. 

Mating  2. 

Ear- 
length. 

Weight. 

Age. 

lenSh.     Weight. 

Age. 

Parents: 

9247 

d'179 

Mid-parental 
Offspring: 

9635 

9636 

9637 

0^638 

^^639 

mm. 

152 
210 
181 

170 

183 
180 
170 
184 

gms. 

3.290 
3.410 
3.350 

1,740 
1,620 
1,990 
1,630 
1,505 

Adult. 
Do. 
Do. 

20  weeks. 

19  weeks. 

20  weeks. 
20  weeks. 
20  weeks. 

Parents : 

9247 

c?3i9 

Mid-parental 
Offspring: 

9731 

9732 

9733 

9734 

(5^736 

c?737 

9738 

mm. 

152 
200 
176 

170 
170 
160 
173 
165 
160 

171 

gms. 

3.290 

3.955 
3,622 

1.970 
2,100 
2,020 
1,710 

r.785 
1.700 
2,020 

Adult. 
Do. 
Do. 

20  weeks. 
Do. 
Do. 
Do. 
Do. 
Do. 
Do. 

Male  248  was  mated  with  three  different  short-eared  females,  none  of 
which  was  nearly  related  to  him.     The  results  are  shown  in  table  15. 

In  mating  i,  c?  248  gives  a  result  like  that  which  he  had  given  when 
mated  with  his  sister  (9  247).  All  but  i  of  the  6  young  fell  below  the  mid- 
parental  ear-length  by  from  8  to  19  mm.  The  shortest-eared  one  had 
exactly  the  same  ear-length  (115  mm.)  as  the  short-eared  parent,  a  result 
unparalleled  elsewhere  in  these  experiments  except  in  one  case,  presently 
to  be  noticed.  The  shortness  in  this  case  can  not  be  attributed  to  the  poor 
condition  or  small  size  of  the  individual,  for  it  was  the  largest  rabbit  but 
one  in  the  litter,  a  position  which  it  maintained  throughout  the  growth 
period.  Apparently  this  individual  represents  an  extreme  variate  of  a 
fluctuating  group.  The  extreme  range  of  variation  in  this  litter  was  23 
mm.;  the  difference  between  the  parents,  38  mm. 


EAR-SIZE 


27 


Mating  2  shows  a  result  more  nearly  normal.  Two  individuals  exceed 
the  mid-parental  ear-length  by  3  mm.,  2  fall  short  of  it  7  mm.,  and  i  by 
9  mm.     The  total  range  of  variation  is  12  mm. 

In  mating  3,  which  jjroduced  2  litters  of  young,  the  variations  are  again 
chiefly  below  the  mid-parental  ear-length,  but  to  no  greater  extent  than 
we  might  expect,  in  view  of  the  difference  in  age  between  parents  and 
children,  when  measured.  In  litter  i  the  deviations  are  —2,  +3,  —4, 
and  —2  mm.,  a  nearly  normal  result;  but  in  litter  2,  four  individuals 
show  a  deviation  of  —7  mm.,  and  one  a  deviation  of  +3  mm.  The  range 
in  htter  i  is  7  mm.;  in  Utter  2,  10  mm.  There  is  no  evidence  of  hetero- 
geneity among  the  gametes. 

Table  15. 


Mating  i. 

Mating  3. 

Ear- 
length. 

Weight. 

Age. 

Ear- 
length. 

Weight. 

Age. 

Parents : 

9255 

C?248 

Mid-parental 
Offspring: 

9619 

9620 

C?62I 

9622 

d'624 

(^625 

mm. 

"5 
153 
134 

125 
120 

138 
"5 
126 

125 

gms. 

2,650 
3.930 
3.290 

1,820 

1.750 
1,700 
1,885 
1,720 
1,910 

Adult. 
Do. 
Do. 

19  weeks. 

20  weeks. 
Do. 
Do. 

19  weeks. 

20  weeks. 

1 

Parents : 

9389 

(^248 

Mid-parental 
Offspring: 
Litter  i  — 

1          9653 

9654 

?655 

9656 

Litter  2  — 

c^793 

9794 

9795 

c?796 

d'797 

mm. 

los 

153 
129 

127 
132 
125 

127 

120 
120 
120 
130 
120 

gms. 

3,090  > 
3.930 

1.465 

1.845 
1,840 

1.745 

1,700 
1.740 
1,860 
1,690 
1,770 

Adult. 
Do. 
Do. 

20  weeks. 
Do. 
Do. 
Do. 

20  weeks. 
Do. 
Do. 
Do. 
Do. 

Mating  2. 

Parents : 

9  269 

6^248 

Mid-parental 
Offspring: 

9726 

9727 

c?728 

6^729 

?730 

92 

153 
122 

"5 
"3 
"5 
125 
125 

1.935 
3.930 
2,932 

1.730 
1,685 
1.695 
1,500 
1.930 

Adult. 
Do. 
Do. 

20  weeks. 
Do. 
Do. 
Do. 
Do. 

•  At  20  weeks. 

Some  other  matings  which  fall  in  the  category  of  cross  6,  and  consti- 
tute a  second  generation  (Fj)  from  cross  3,  are  included  in  table  16. 

The  statistics  contained  in  table  16  are  not  very  satisfactory  because 
the  observations  are  made  at  such  dilTerent  ages,  and  because,  in  one  case 
at  least,  that  of  (?38i,  a  remarkable  increase  in  ear-length  is  recorded 
subsequent  to  the  age  of  18  weeks.  For  observations  made  at  the  same 
age,  however,  the  variability  in  ear-length  is  considerable.  Tlie  range 
of  variation  in  mating  i,  litter  i,  is  17  mm.;  in  litter  2  it  is  15  mm.;  in  mating 
2,  it  is  18  mm.     In  generation  F,,  cross  3,  the  range  of  variation  was  only 


28 


INHERITANCE    IN   RABBITS 


slightly  less,  viz,  14  mm.  So  far,  then,  as  table  16  is  concerned,  we  get 
no  clear  evidence  of  heterogeneity  among  the  gametes  formed  by  the 
cross-breds  produced  by  cross  3. 

Table  16. 


Mating  i. 

Mating  2. 

Ear- 
length. 

Weight. 

Age. 

Ear- 
length. 

Weight. 

Age. 

Parents: 

9i7S(pI-3, 
fig.  10)  . . . 

c?i76 

Mid-parental 
Offspring: 
Litter  i  — 

c?37S 

d^376 

c?38i   (pl.  3, 
fig.  12)  ..  . 

Litter  2  — 

c?759 

c?76o 

9761 

mm-. 

170 
166 
168 

1 60' 
172I 

i  '"'.  . 
]  168 

180 
170 

185 

gms. 

4,305 
4,130 
4,218 

2,960 

2,975 
2,800 

3,125 
3,800 

2,220 
2,200 
2,410 

Adult. 
Do. 
Do. 

6  months. 

Do. 
22  weeks. 
6  months. 
I  year. 

20  weeks. 
Do. 
Do. 

Parents: 

9178 

c?i76 

Mid-parental 
Offspring: 

9471 

9472 

d'474 

9475 

9476 

(^480 

mm. 

170 
166 

168 

147 
168 

163 
150 
150 
150 

gms. 

4,070 
4,130 
4,100 

1,470 
1,865 
1,733 
1,515 
1,750 
1,610 

Adult. 
Do. 
Do. 

17  weeks. 

Do. 

Do. 

Do. 

Do. 
15  weeks. 

1 18  weeks. 


The  female  half-blood  lop  175  (plate  3,  fig.  10)  produced  by  cross  3 
was  mated  with  the  three-quarter-blood  lop  c?437  (cross  5),  and  produced 
a  litter  of  5  young,  the  character  of  which  is  shown  in  table  17. 


Table  i 

7- 

Ear- 
length. 

Weight. 

Age. 

Parents: 

9i7S 

c?'437 

Mid-parental  .... 
Offspring: 

9847 

mm. 

170 
I184 
/  200 

185 

176 
188 
180 

174 
190 

gms. 

4,305 
2,550 
3,140 
3,722 

1,920 
1,800 
1,970 

1,750 
1,790 

Adult. 
20  weeks. 
30  weeks. 
Adult. 

18  weeks. 

17  weeks. 
Do. 

16  weeks. 

18  weeks. 

9  848 

Q  840 

c?'8<;i 

Ji8<:2 

These  young  early  (16  to  18  weeks)  attained  a  large  size,  indicating 
conditions  favorable  for  growth.  In  ear-length  they  fluctuated  about 
the  mid-parental  condition,  which  was  exceeded  by  2  individuals,  while 
3  fell  short  of  it.  All  had  ear-lengths  intermediate  between  those  of  their 
respective  parents.  The  range  of  variation  among  them  at  18  weeks 
was  14  mm.,  exactly  the  same  as  in  cross  3,  from  which  the  mother  was 
derived.  The  greatest  deviations  from  the  mid-parental  were  —11  (at 
16  weeks)  and  -I-5  (at  18  weeks).     No  e^^dence  is  afforded  of  unusual 


EAR-SIZB 


29 


heterogeneity  among  the  gametes  of  either  parent,  although  both  were 
cross-bred    individuals. 

Females  175  and  178  (cross  3)  were  also  used  in  back-crosses  with  a 
lop-eared  male  (179),  resulting  in  the  production  of  three-quarter-blood 
lops.     The  character  of  these  young  is  shown  in  table  18. 

Table  18. 


Mating  i. 

Mating  2. 

Ear- 
length. 

Weight. 

Age. 

Ear- 
length. 

Weight. 

Age. 

Adult. 
Do. 
Do. 

20  weeks. 
26  weeks. 
19  weeks. 
26  weeks. 

Do. 

Do. 

Parents : 

9175 

^"179 

Mid-parental 
Offspring: 

9487 

(5*491 

(5'492 

c?493 

mm. 

170 
210 
190 

182 

182 

I  190 

/  200 

170 

gms. 

4.305 
3.410 

3.857 

2.740 
1.795 
2.035 
3.330 
1,900 

Adult. 
Do. 
Do. 

6  months. 
16  weeks. 

Do. 
Adult. 
16  weeks. 

[  Parents: 

!      9178 

6^179 

Mid-parental 
Offspring: 

9546 

?547 

(5*548 

SS49 

9550 

9552 

mm. 

170 
210 
190 

190 
'85 
»9S 
193 
200 

185 

gms. 

4,070 
3.410 
3.740 

2,242 
3.«6o 
2,320 
2,900 
2,460 
2,360 

The  data  for  mating  i  are  incomplete;  but  those  recorded  for  mating  2 
are  entirely  satisfactory.  They  show  a  total  range  of  variation  in  ear- 
length  at  26  weeks  of  15  mm.  which  is  very  similar  to  that  found  in  cross 
3,  by  which  the  mother  w^as  produced.  The  greatest  minus  deviation 
from  the  mid-parental  ear-length  is  5  mm.;  the  greatest  plus  deviation, 
10  mm.;  the  average  deviation,  4.6  mm.  The  nearest  approximation  to 
the  short-eared  parent  is  15  mm.,  to  the  long-eared  parent  10  mm.  The 
inheritance  may  fairly  be  described  as  blending,  with  no  evidence  of  seg- 
regation in  Fj. 

The  half-blood  lop  (^176  was  also  employed  in  a  back-cross  with  his 
mother,  the  Belgian  hare,  9431  (plate  3,  fig.  9).  Table  19  shows  the 
result  obtained. 

Table  19. 


Ear- 
length. 

Weight. 

Age. 

Parents: 

94^1 

mm. 

118 
166 
142 

145 
142 

135 
135 
128 

gms. 

3.400 
4.130 
3.765 

2,660 

2.87s 
2,520 
2,690 
2,180 

Adult. 
Do. 
Do. 

27  weeks. 
Do. 
Do. 
Do. 
Do. 

(?i76 

Mid-parental .... 
Offspring : 

(5*52o 

9  C2I 

(5^522 

9  ea-i 

9  C24 

Average 

137 

2,581 

30 


INHERITANCE    IN   RABBITS 


The  offspring  fluctuate  in  ear-length  about  the  mid-parental  condi- 
tion; 2  of  them  exceed  it,  3  fall  short  of  it.  The  minus  deviations,  how- 
ever, are  greater  than  the  plus  ones,  precisely  as  in  lop-eared  rabbits  bred 
inter  se  (p.  17).  The  range  of  variation  is  17  mm.,  which  is  greater 
than  that  occurring  among  short-eared  rabbits,  but  less  than  that  occur- 
ring among  lop-eared  rabbits. 

Another  son  of  9431,  own  brother  to  J' 176,  was  Ukewise  mated  with 
his  mother.  This  male  (177,  cross  3)  had  ears  10  mm.  shorter  than  those 
of  his  brother  (d'176).  The  mid-parental  ear-length,  accordingly,  was 
only  137  mm.  Two  young  only  were  reared  to  an  age  of  20  weeks,  and 
each  of  them  had  an  ear-length  of  125  mm. 

Further  tests  of  the  half-blood  lop  females  175  and  178  are  afforded 
by  the  crosses  recorded  in  table  20,  with  a  related  three-quarter-blood 
lop  male  (319)  produced  by  cross  4. 

Table  20. 


Mating  i. 

Mating  2. 

Ear- 
length. 

Weight. 

Age. 

Ear- 
length. 

Weight. 

Age. 

Parents: 

9i7S 

c?3i9 

Mid-parental 
Offspring : 

9674 

c?67S 

d'677 

c?678 

mm. 

170 

200 
185 

185 

175 
192 
180 

gms. 

4,305 
3,955 
4,130 

2,400 

2,350 
2,410 
1,420 

Adult. 
Do. 
Do. 

20  weeks. 
Do. 
Do. 
Do. 

Parents: 

9178 

(5^319 

Mid-parental 
Offspring: 
Litter  i  — 

?66o 

6^661 

Litter  2  — 

0^754 

c?75S 

mm. 

170 
200 
185 

195 
191 

150 
162 

gms. 

4,070 

3,955 
4,012 

2,100 
2,750 

1,460 
1,860 

Adult. 
Do. 
Do. 

20  weeks. 
Do. 

19  weeks. 
18  weeks. 

The  young  produced  by  mating  i  are  all  intermediate  in  ear-length 
between  their  parents.  One  ( 9  674)  exactly  attains  at  20  weeks  of  age 
the  mid-parental  ear-length,  a  second  ( s  678)  would  doubtless  have  done 
so  had  he  not  fallen  into  bad  condition  at  about  13  weeks  of  age.  Previous 
to  that  he  had  been  one  of  the  largest  and  longest-eared  rabbits  in  the  htter. 
Of  the  remaining  2,  both  of  which  developed  normally  and  were  of  large 
size  at  20  weeks  of  age,  one  exceeded  the  mid-parental  ear-length  by  7  mm. 
and  the  other  fell  10  mm.  short  of  it,  approaching  to  within  5  mm.  of  the 
ear-length  of  the  short-eared  parent.  The  range  of  variation  (17  mm.)  is 
not  excessive,  and  the  result  may  be  described  as  a  fully  normal  blend,  with 
no  indication  of  heterogeneity  among  the  gametes  of  the  cross-bred  parents. 

Mating  2  yielded  2  htters  very  different  in  character  and  illustrating 
rather  strikingly  the  influence  of  external  conditions  on  growth.  Litter 
I  consisted  of  2  young  only.  They  were  born  in  summer  and  developed 
under  optimum  conditions  as  regards  food  supply.  At  20  weeks  of  age 
they  had  attained  large  size  and  had  ear-lengths  exceeding  by  6  and  10 


EAR-SIZE 


31 


mm.  respectively  the  mid-parental  ear-k-rif^th.  Litter  2,  on  the  other  hand, 
was  born  in  the  winter.  It  consisted  originally  of  8  individuaLi.  The 
2  weakest  ones  in  the  litter  died,  one  previous  to,  the  other  subsequent 
to  weaning.  The  4  largest  ones  were  stolen,  leaving  2  survivors,  cT  754 
and  c?  755,  both  of  which  when  last  measured  gave  evidence  of  having 
been  ])ermanently  stunted  in  size  and  ear-length  by  the  hard  conrlitions 
under  which  they  had  developed.  They  are  too  abnormal  to  throw  any 
light  on  the  inheritance  of  ear-length  in  this  cross. 

MATINGS   OF   THREE-QUARTER-BLOOD    LOPS. 

The  male  319,  employed  in  the  matings  last  described,  was  also  used 
in  crosses  with  short-eared  rabbits  and  w:th  a  three-quarter-blood  lop, 
his  sister.  The  results  of  the  crosses  with  short-eared  females  are  shown 
in  table  21. 

Table  21. 


The  7  young  produced  b\'  mating  i  iluctuate  about  the  mid-parental 
condition  of  ear- length.  The  greatest  minus  variation  is  11  mm.,  the 
greatest  plus  variation  8  mm.,  giving  a  total  range  of  variation  of  19  mm. 
This  is  not  large,  considering  that  the  difference  between  the  parents  is 
95  mm.  The  greatest  deviation  from  the  mid-parental,  11  mm.,  is  36 
mm.  removed  from  the  nearest  parental  ear-length,  that  is,  it  is  less  than 
one-third  as  great  as  the  least  deviation  from  either  parent.  The  inheri- 
tance is  unmistakably  blending.  Even  more  clearly  is  this  the  case  in 
mating  2.  The  parents  dilTer  in  ear-length  by  100  mm.  The  young  are 
all  almost  exactly  intermediate.  The  entire  range  of  variation  in  the  6 
young  is  only  5  mm.,  while  the  nearest  approximation  to  the  car-length 
of  either  parent  is  nine  times  this  amount.  A  better  example  of  fully 
blending  inheritance  can  scarcely  be  imagined.  In  neither  mating  do 
we  get  evidence  of  heterogeneity  among  the  gametes  formed  by  the  three- 
quarter-blood  father  (,J3i9). 


32 


INHERITANCE   IN   RABBITS 


It  is  of  interest  to  note  that  both  the  mothers  employed  in  matings  i 
and  2  were  employed  also  in  cross  2,  with  the  lop  $  179.  In  that  case, 
also,  they  gave  a  distinctly  and  fairly  uniform  blending  result. 

Male  319  was  mated  also  with  his  sister  (9322),  producing  a  litter  of 
only  2  young.  These  closely  resembled  their  parents  in  ear-character. 
(See  table  22.) 

Table  22. 


Ear- 
length. 

Weight. 

Age. 

Parents: 

Q  122 

mm. 

195 

200 

197-5 

208 
190 

gms. 

4.450 

3.955 
4,202 

2,680 
1.950 

Adult. 
Do. 
Do. 

20  weeks. 
16  weeks. 

(^319 

Mid-parental .... 
Offspring : 

9  770 

9780 

The  latest  measurement  recorded  for  one  of  them  ( 9  780)  was  made 
at  16  weeks  of  age,  but  aheady  she  had  attained  an  ear-length  of  190 
mm.  At  maturity  she  would  doubtless  have  equaled  or  exceeded  the  mid- 
parental  ear-length.  The  other  one  (9  779)  did  exceed  the  mid-parental 
ear-length  at  20  weeks  of  age  by  more  than  10  mm.,  and  she  exceeded  by 
8  mm.  the  ear-length  of  the  long-eared  parent.  Her  size  also  at  20  weeks 
of  age  was  very  large,  viz,  2,680  grams.  This  unusually  great  plus  vari- 
ation was  doubtless  due  in  part  to  extremely  favorable  conditions  during 
the  growth  period,  especially  during  the  period  of  lactation.  During 
that  period  the  mother's  milk  was  divided  among  3  young  only,  but  i  of 
these  died  soon  after  the  young  were  weaned.  At  the  last  measurement 
recorded,  its  ear-length  was  a  httle  less  than  that  of  9  780,  while  in  size 
it  was  inferior  to  both  9  779  and  9  780. 

Table  23. 


Ear- 
length. 

Weight. 

Age. 

Parents : 

Q  722 

mm. 

195 
210 

gms. 

4.450 
3.410 
3.930 

2,460 

2,78s 
2,220 
2.510 
1,970 
2,86s 
2,080 

2.235 

Adult. 
Do. 
Do. 

20  weeks. 
25  weeks. 
20  weeks. 
25  weeks. 
20  weeks. 
33  weeks. 
20  weeks. 
25  weeks. 

r?l70 

Mid-parental .... 

Offspring: 

Q  c8o 

202.5 

205 
210 
200 
205 

195 
200 
190 
195 

Q  coo 

rT'coi 

9592 

The  same  three-quarter-blood  female  (322)  which  was  mated  with 
c?3i9  (table  22)  was  mated  also  with  the  lop  s  179,  producing  a  litter  of 
seven-eighth-blood  lops.     (See  table  23.)     Two  of  the  4  young  reared  had 


EAR-SIZE 


33 


at  25  weeks  of  age  ear-lengths  identical  with  those  of  the  respective  parents, 
viz,  of  195  and  210  mm.  The  other  two  had  intermediate  ear-lengths 
of  200  and  205  mm.  resjjectively.  This  is  a  fully  normal  blending  result. 
The  total  range  of  variation  is  15  mm.  In  both  car-length  and  size  the 
young  are  similar  to  those  produced  by  the  mating  with  (^319  (table  22). 

Cross  7. —  Quarter-blood  Lop  Fem/Vle  X  Short-eared  Male. 

Three  different  quarter-blood  lop  females,  521,  522,  and  524  (table  18), 
produced  by  a  mating  of  the  Belgian  hare  with  her  son  (d"  176),  were 
mated  with  a  son  of  the  same  Belgian  hare  by  an  unrelated  short-eared 
male.    (See  table  14.)   The  outcome  of  these  matings  is  shown  in  table  24. 

Table  24. 


Mating  i. 

Mating  3. 

Ear- 
length. 

Weight. 

Age. 

Ear- 
length. 

Weight. 

Age. 

Parents: 

9521 

(^235 

Mid-parental 
Offspring: 

dSog 

c?8io 

98ii 

9813 

c?8i4 

9815 

mm. 

142 
no 
126 

"5 
1 30 

"5 
125 
126 

gms. 

2,875 
2,945 
2,910 

2,080 
1,890 
1,820 

2,03s 
1,960 

2,150 

27  weeks. 
Adult. 

20  weeks. 
Do. 
Do. 
Do. 
Do. 
Do. 

Parents: 

9524 

c?235 

Mid-parental 
Offspring: 

cJ;834 

<^835 

9836 

9837 

9838 

mm. 

128 
no 
,19 

"5 
"S 
121 
116 
134 

gms. 

2,160 

2,945 
2.552 

1,600 
1,700 
1,820 

1,450 
2,000 

27  weeks. 
Adult. 

20  weeks. 
Do. 
Do. 
Do. 
Do. 

Mating  a. 

Parenu: 

9522 

(5*235 

Mid-parental 
Offspring: 

9823 

d'824 

13s 

no 

122.5 

125 
125 

2,520 

2,945 
2,732 

2,300 
2,250 

27  weeks. 
Adult. 

20  weeks. 
Do. 

The  offspring  show,  as  regards  ear-length,  a  rather  wide  range  of  vari- 
ation, 20  mm.,  which  is  nearly  two-thirds  of  the  difference  in  ear-length 
between  the  parents.  The  average  ear-length  of  the  offspring  corresponds, 
in  each  Utter,  closely  with  the  mid-parental  ear-length,  the  j^lus  and  minus 
deviations  being,  except  in  mating  3,  about  equal  in  number  and  amount. 
In  mating  i,  3  of  the  6  young  have  api)roximately  the  mid-parental  ear- 
length,  but  2  show  minus  deviations  of  6  and  11  mm.  respectively,  and  i 
shows  a  plus  deviation  of  9  mm. 

The  2  young  produced  by  mating  2  were  of  large  size  at  20  weeks  of 
age,  indicating  conditions  of  nutrition  above  the  average.  The  ear-length 
of  each  exceeds  by  2,5  mm.  the  mid-parental  ear-length. 

The  5  young  produced  by  mating  3  show  2  minus  deviations  of  3  and  4 
mm.  res])cctively,  and  3  j)lus  deviations  of  2,  6,  and  15  mm.  respectively. 


34 


INHERITANCE   IN   RABBITS 


Cross  8.  —  Quarter-blood  Lop  x  Three-quarter-blood  Lop. 

A  single  mating  of  this  sort  produced  a  litter  of  3  young,  all  very  sim- 
ilar and  close  to  the  mid-parental  ear-length.  (See  table  25.)  The 
observations  were  discontinued  when  the  young  were  14  or  16  weeks  old, 
but  the  mid-parental  ear-length  of  the  parents,  when  they  were  20  weeks 
old,  had  already  been  closely  approximated.  The  deviations  were  —2, 
—  5,  and  —5  mm.  If  growth  progressed  normally  from  the  age  of  14  or 
16  weeks  on,  they  would  surely  have  attained  the  adult  mid-parental 
ear-length,  viz,  167.5  mm. 


Table 

25- 

Ear- 
length. 

Weight. 

Age. 

Parents : 

9  C2-* 

mm. 

\  130 
1  13s 
1  19s 
1  200 

162.5 
1  167.5 

160 

157 
157 

gms. 

1,930 
2,690 
2,540 
3,330 

2,235 
3,010 

1,450 
1,300 
1,450 

20  weeks. 
27  weeks. 
20  weeks. 
43  weeks. 
20  weeks. 
Nearly  grown. 

14  weeks. 
14  weeks. 
16  weeks. 

r^AQ  2 

Mid-parental.  . .  . 

Offspring: 

884 

885 

886 

LIMITATIONS    OF    THE    DATA    STUDIED. 

In  attempting  to  draw  conclusions  from  the  statistics  presented  in  the 
foregoing  pages,  one  must  bear  in  mind  certain  of  their  Hmitations  and 
imperfections. 

(i)  Ear-length  is  modified  to  some  extent  by  external  conditions.  If  the 
young  rabbit  is  well  nourished  up  to  the  age  of  20  weeks,  its  ear-length 
will  be  greater  than  if  it  is  poorly  nourished,  other  conditions  being  equal. 
While  we  have  attempted  to  give  our  rabbits  the  best  of  care  at  all  sea- 
sons, it  is  inevitable  that  the  quality  of  food  supplied  at  different  seasons 
of  the  year  should  vary,  and  with  variation  in  the  quaUty  of  the  food  goes 
variation  in  the  growth  rate.  This  renders  it  difficult  to  compare  with 
each  other,  as  regards  ear-character,  rabbits  reared  at  different  seasons 
of  the  year.  But  it  has  been  impossible  for  us  to  rear  enough  rabbits  at 
any  one  season  to  afford  adequate  material  for  comparisons.  Hence  we 
are  forced  to  utihze  material  produced  at  different  seasons  of  the  year. 

(2)  Size  of  litter  is  of  some  consequence  in  determining  the  growth  rate 
of  a  rabbit.  If  there  are  several  young  in  a  litter  each  gets  a  smaller 
amount  of  food  during  the  period  of  lactation  than  it  would  have  received 
had  the  litter  been  smaller.  Our  material,  however,  is  not  extensive  enough 
to  allow  us  to  institute  comparisons  merely  between  litters  of  substantially 
the  same  size. 


EAR-SIZE  35 

(3)  It  is  the  belief  of  fanciers  that  a  warm,  moist  atmosjihere,  during 
the  period  of  active  growth  of  the  ears,  favors  the  attainment  of  large 
ear-size.  This  view  we  have  not  been  able  to  juit  to  an  experimental 
test,  but  we  are  inclined  to  think  that  the  temperature  and  humidity  arc 
much  less  important  factors  than  abundant  food  supply. 

(4)  Rabbits  of  the  small,  short-cared  races  have  a  shorter  growth  period 
than  the  larger  races.  Their  ears  are  more  likely  to  be  full-grown  at  20 
weeks  of  age  than  are  those  of  loj)-eared  rabbits.  Therefore,  in  comi)aring 
rabbits  of  ditTerent  ancestry  at  the  same  age,  say  20  weeks,  one  is  in  dan- 
ger of  underestimating  the  ear-length  of  the  larger-sized  rabbit. 

(5)  A  cross  between  rabbits  of  entirely  different  races  is  likely  to  result 
in  young  of  unusual  vigor,  which  causes  them  to  attain  a  greater  weight 
and  ear-length  than  the  hereditary  constitution  of  either  race  by  itself 
would  result  in.  This  is  illustrated  notably  in  cross  3,  page  20.  Supe- 
rior size  or  ear-length,  induced  by  crossing,  we  should  not  e.xpcct  to  be  per- 
manent in  later  generations. 

(6)  Disease  frequently  interrupts  the  orderly  progress  of  a  growth-curve 
and  necessitates  the  omission  altogether  of  certain  series  of  observations. 

CONCLUSIONS. 

Notwithstanding  these  limitations,  which  manifestly  restrict  the  scope 
of  our  conclusions,  certain  generaUzations  are  clearly  justified. 

(i)  A  cross  between  rabbits  differing  in  ear-length  produces  offspring 
with  ears  of  intermediate  length,  varying  about  the  mean  of  the  parental 
ear-lengths. 

(2)  It  is  immaterial  whether  the  larger  parent  was  father  or  mother; 
the  result  is  the  same  in  either  case.  As  regards  ear-length,  then,  we  may 
say,  reciprocal  crosses  give  the  same  result.  This  shows  that  ear-size  is 
a  character  inherited  with  equal  intensity  through  father  or  mother. 

(3)  A  study  of  the  offspring  of  the  primary  cross-breds  shows  the  blend 
of  the  parental  characters  to  be  permanent.  No  reappearance  of  the  grand- 
parental  ear-lengths  occurs  in  generation  F2,  nor  are  the  individuals  of 
that  second  generation  as  a  rule  more  variable  than  those  of  the  first  gener- 
ation of  cross-breds.  Fig.  3  shows  the  most  extreme  case  of  "scatter"  in 
F2  that  we  have  observed.  Yet  the  variation  in  this  case  is  no  greater 
than  among  the  young  of  lop-eared  rabbits  bred  inler  se. 

(4)  The  extreme  range  of  variation  in  ear-length  among  short-eared 
rabbits  is  about  10  mm.;  in  lop-eared  rabbits  it  is  two  or  three  times  as 
great,  or  from  20  to  30  mm.  Among  rabbits  produced  as  crosses  of  vari- 
ous sorts  between  short-eared  and  lop-eared  rabbits  the  range  of  varia- 
tion in  ear-length  is  mostly  intermediate  in  amount. 

(5)  The  form  of  the  growth-curve  for  ear-length  from  the  age  of  2  weeks 
on  is  convex  upward,  indicating  a  steady  diminution  in  the  daily  growth 
increment. 


PART    II.  — WEIGHT. 


Our  statistics  for  size  inheritance  arc  not  very  satisfactory,  because  we 
were  unable  to  keep  any  considerable  number  of  rabbits  until  they  were 
full  grown,  owing  to  the  smallness  of  our  breeding  room,  so  that  a  large 
number  of  weighings  of  adults  is  not  available  for  jjurposes  of  compari- 
son. But  the  size  of  a  growing  rabbit  varies  greatly  with  the  character 
of  its  food,  and  this  in  turn  is  dependent  upon  a  variety  of  conditions 
which  it  was  not  possible  for  us  fully  to  control.  A  com[)arison  of  the 
weights  of  growing  rabbits  at  correspomh'ng  ages  is,  therefore,  not  alto- 
gether satisfactory,  yet  it  is  the  best  material  we  have. 

In  tables  i  to  25  the  latest  available  weighing,  or  the  heaviest  weight, 
is  recorded  for  each  rabbit.  But  since  the  weighings  there  recorded  were 
made  at  very  different  ages,  it  is  necessary  to  select  some  particular  age 
at  which  to  make  comparisons.  The  age  of  18  weeks  has  been  selected, 
because  the  weighings  for  that  age  are  most  numerous. 

In  table  26  are  shown  the  average  weights,  at  18  weeks  of  age,  of 
different  lots  of  rabbits,  each  lot  containing  those  of  like  ancestry.  The 
number  of  individuals  in  each  lot  is  also  shown  in  the  table,  as  well  as 
the  greatest  range  of  variation  in  weight  found  in  any  litter  of  each  lot. 
The  statistics  in  table  26  are  fullest  for  those  crosses  (left  section)  in  which 
ordinary  short-eared  rabbits  were  concerned.  The  average  weight  of  such 
rabbits,  in  a  lot  of  17  individuals,  is  seen  to  be  1,412  grams.  For  lop- 
eared  rabbits  it  is  something  over  1,743  grams,  the  weight  given  in  the 
table  from  observations  on  2  rabbits.  This  weight,  however,  has  been 
exceeded  at  14  weeks  of  age  by  a  majority  of  the  lop-eared  rabbits  which 
we  have  reared,  so  that  it  is  certainly  too  low. 

The  lots  of  rabbits,  partly  of  short-eared,  partly  of  lop-cared  ancestry, 
have  intermediate  weights,  the  weight  tending  to  increase  with  increase 
in  the  proportion  of  lop  blood.  The  variability  (range)  in  weight,  which 
was  found  to  be  twice  as  great  in  lop-eared  as  in  short-eared  rabbits,  is 
intermediate  in  the  cross-bred  lots,  increasing  with  increase  in  the  propor- 
tion of  lop  blood. 

Both  the  position  of  the  average  for  each  lot,  and  the  amount  of  variation 
within  it,  indicate  that  weight-inheritance,  like  the  inheritance  of  ear- 
size,  is  blending  in  character.  Neither  dominance  nor  segregation  in 
the  Mendelian  sense  are  recognizable. 

The  Belgian  hare  crosses  and  mixed  crosses,  recorded  in  the  last  sec- 
tions of  tabic  26,  show,  in  general,  results  similar  to  those  given  by  the 
crosses  with  short-eared  rabbits,  but  man\-  of  the  averages  are  less  reliable 

37 


38 


INHERITANCE    IN    RABBITS 


because  based  on  too  few  individuals  (in  4  cases,  a  single  litter  each  time). 
The  Belgian  hare  was  heavier  than  the  short-eared  stock  used,  and  it 
will  be  seen  that,  in  all  cases,  her  descendants  exceed  in  size  animals  of 
the  short-eared  series  having  a  like  amount  of  lop  blood.  Further,  a 
mixture  of  short  and  Belgian  blood  tends  to  produce  a  rabbit  intermedi- 
ate in  weight  between  those  of  the  short  and  of  the  Belgian  series,  respec- 
tively. (See  table  26,  right  section.)  All  these  observations  confirm  the 
idea  that  body-weight  is  a  character  blending  in  its  inheritance. 

Table  26.  —  Size  at  18  weeks  of  age  of  rabbits  of  different  proportions  of  lop 
^^  blood,"  from  crosses  with  short-eared,  with  the  Belgian  hare,  or  with  both. 


Lop  blood. 

Short-eared. 

Belgian  hare. 

Mixed. 

Average 
weight. 

Max. 
varia- 
tion in 
weight. 

Individ- 
uals ob- 
served. 

Average 
weight. 

Max. 

varia- 
tion in 
weight. 

Individ- 
uals ob- 
served. 

Average 
weight. 

Max. 
varia- 
tion in 
weight. 

Individ- 
uals ob- 
serv    e. 

None 

gms. 
1,412 

1.592 

1.463 
1,700 

1.585 
1,652 

1.743* 

gms. 
315 

345 

315 
420 
420 

495 
8oo2 

17 
20 

17 
18 

35 
17 

7 

gms. 
1,788 

2,076 

1.965 
1,940 

1.954 
1.936 

gms. 
.... 

627 

260 

820 
890 
890 
590 

5 

4 

.... 

12 
10 

gms. 

1.791 
1,580 
1,888 

1.754 

gms. 

490 
205 

575 
420 

13 

4 

14 

II 

One-eighth 

One-fourth 

Three-eighths.  .  .  . 
One-half: 

Gen.  I 

Gen.  2 

Both 

Three-fourths : 

Gen.  I 

Gen.  2 

Both 

Seven-eighths    . .  . 
All 

4 

1  This  is  certainly  too  low,  for  in  litter  70,  table  2,  mating  3,  it  was  surpassed  by  three  of  the 
five  rabbits  of  the  litter,  already  at  14  weeks  of  age.  The  average  given  (1,743)  is  for  the  two  ani- 
mals, 6^179  and  9  180  (table  2,  mating  i). 

2  At  14  weeks. 

When  the  parents  differ  in  size,  the  young  are  clearly  of  intermediate 
size,  but  our  observations  are  too  incomplete  to  show  in  most  cases  whether 
the  size  is  midway  between  that  of  the  respective  parents  or  not.  Prof. 
W.  C.  Sabine  has  kindly  pointed  out  that  if  linear  dimensions  give  a  mid- 
parental  condition  (the  mean  of  the  respective  parental  conditions),  then 
we  should  expect  the  weights  to  be  less  than  the  mean  of  the  parental 
weights,  provided  the  proportions  of  parts  are  the  same  in  all  cases.  But 
the  proportions  of  the  parts  are  different  in  the  two  parents,  when  rabbits 
of  different  size  are  mated  with  each  other,  and  the  proportions  in  the  off- 
spring are  unmistakably  intermediate  between  those  of  the  respective  parents. 

This,  perhaps,  accoimts  for  some  of  the  peculiarities  observed  in  com- 
paring weights  of  the  rabbit  c?  248  with  those  of  his  parents,  and  with 
the  mid-parental  weights.  All  three  rabbits  were  fully  grown  (2  or  more 
years  old)  when  the  observations  were  made,  and  these  are  fairly  com- 
plete.     The  maximum  body-weight  recorded  for  S  248  was  somewhat 


WEIGHT 


39 


in  excess  of  the  mid-parental  (table  27),  but  since  he  shows  a  less  per- 
centage of  bone  to  total  body-weight  than  either  i)arent  it  is  j)robable 
that  the  excess  is  due  in  part  at  least  to  some  tem[)orary  condition  (fat- 
ness). If  he  showed  a  mid-j)arental  ])ercentage'  of  bone  to  body-weight, 
and  this  would  possibly  be  the  case  if  all  3  rabbits  had  been  in  like  con- 
dition, as  regards  fatness,  then  his  weight  should  be  something  less  than 
the  mid-parental  weight,  or  about  3,326  grams,  instead  of  the  mid-parental 
weight  (3,800)  or  the  observed  maximum  weight  (3,930).  But  in  the 
absence  of  more  extensive  observations,  we  can  not  be  certain  that  the 
percentage  ratio  of  bone  to  body-weight  is  a  mid-percentage.  The  ques- 
tion must  remain  an  o])en  one  until  further  data  can  be  accumulated. 


Table  27. 


Relation  between  hone-weights  and  total  body-weight  in  the 
rabbit  <3  248  and  in  his  parents. 


Bone- 
weight. 

Bodv- 
weight. 

Per  cent 
bone- 
weight. 

Old  9lop   

J>4<;     

gms. 
77-15 
39-35 
58-a5 
49-7 

1 
4,600               1.67 

Mid-parental 

Son,  (^2^9> 

3,800 
3.930 

.... 
1.36 

As  regards  bone-size,  however,  we  can  reach  more  satisfactory  conclu- 
sions, for  this  character  is  unaffected  by  temporary  conditions  of  the  tlesh. 
In  table  29  are  recorded  bone  measurements  of  this  family  of  rabbits, 
which  show  the  measurements  of  the  son  ( <S  248)  to  be  close  to  the  mid- 
parental  as  regards  both  absolute  measurements  and  j)roportions  of  parts. 

In  tabic  28  arc  recorded  observations  upon  the  weight  and  volume  of 
certain  bones  of  these  same  rabbits.  Both  in  weight  and  in  volume  the 
bones  of  the  son  are  less  than  the  mid-parental.  This  is  what  we  should 
expect  if  the  bones  of  the  son  correspond  with  the  mid-parental  in  linear 
dimensions  and  in  the  proportions  of  parts;  for  linear  dimensions  should 
be  to  each  other  about  as  the  cube  roots  of  the  volumes  and  weights  (i)ro- 
vided  the  specific  gravity  is  alike  in  all  three  cases).-  On  this  hypothesis 
"expected  weights"  and  "expected  volumes"  have  been  calculated  for 
the  son,  and  these  are  entered  in  table  28  in  a  parallel  column,  along  with 
the  observed  weights  and  volumes.  It  will  be  noticed  that  the  "expected" 
uniformly  exceed  the  observed  weights  and  volumes.  The  expected,  to 
be  sure,  is  less  than  the  mid-parental,  but  the  observed  is  still  less.  As 
regards  the  total  weights  of  the  parts  observed,  a  graj^hic  presentation  is 
made  in  fig.  4  of  the  relation  of  mid-parental  to  expected  and  observed. 
The  expected  falls  below  the  mid-parental  by  a  certain  amount,  but  the 

>  The  percentages  given  are  based  upon  the  combined  weights  of  particular  bones,  not  of  all  the 
bones  of  the  body. 

'  A  comparison  of  the  weights  and  volumes  of  corresponding  bones  in  t.iblc  28  indicates  that  the 
specific  gravity  of  the  bones  of  the  son  (d^248)  was  slightly  less  than  that  of  either  parent,  \\z, 
about  1. 19  for  the  son,  i.ao  for  the  mother,  and  i.a6  for  the  father. 


40 


INHERITANCE    IN   RABBITS 


observed  falls  below  the  mid-parental  by  about  three  times  that  amount. 
It  is  in  fact  removed  from  the  mid-parental  only  a  little  less  than  from 
the  weight  of  the  smaller  parent.  It  is  difficult  to  explain  this  extensive 
deviation,  but  it  undoubtedly  exists  and  is  apparently  fairly  uniform, 
though  possibly  the  method  of  computing  the  "expected"  magnitudes  is 
faulty.  The  computation  is  made  in  the  following  way.  The  cube 
root  of  the  weight  for  each  parent  was  found.  These  two  roots  were  then 
added  together,  and  their  half-sum  found,  which  was  then  cubed. 


B 
I 


E 


Fig.  4.  Relation  of  the  bone-weights  of  rabbit  248  to  those  of  his  parents  and  to  the  mid- 
parental  bone-weights.  If  distances  are  laid  off  from  a  point  at  the  left  proportional  to 
the  bone-weights,  they  bear  to  each  other  the  spacial  relations  of  the  following  points : 
A,  bone-weights  of  the  mother;  B,  of  the  father;  D,  of  the  son,  c?  248;  C,  the  mid-parental; 
E,  the  expected  bone-weights  of  the  son. 

Observed  and  expected  deviate  in  the  same  sense  from  the  mid-parental, 
that  is,  are  less,  but  the  uniform  difference  in  amount  between  observed 
and  expected  is  something  requiring  fuller  analysis. 

Table  28.  —  Weights  and  volumes  of  skeletal  parts  of  c?  248  in  relation 

to  those  of  his  parents. 

[Mother,  old  female  lop;  father,  (^45;  son,  (5*248.     Plate  i,  figs.  2,  3,  i.] 


Part. 

Weights  of  bones  (grams). 

Mother. 

Father. 

Mid- 
parental. 

Son. 

Ob- 
served. 

Ex- 
pected. 

Devia- 
tion of 
observed 
from  ex- 
pected. 

Devia- 
tion of 
expected 
from 
mid- 
parental. 

Devia- 
tion of 
observed 
from 
mid- 
parental. 

Humerus    

Femur 

Tibia-fibula 

Innominate  (of  one 

side) 

1 2  ribs  (of  one  side) 

Vertebras: 

I  to  6 

7  to  I  2 

13  to  20  

Total,  I  to  20 . 

Total,    counting 
weight  of  verte- 
bras once  only  . 

5-9 

10.8 

9.1 

8.4 
6-95 

3-2 
5-9 

5-45 

4-1 

3-2 

4-55 
8.35 
7.27 

6.25 
S-07 

3-7 

7-5 
6.1S 

S-4 
3-9 

4.4 
7.8 
7-13 

5-9 
4-7 

-0-7 
-0-3 
-0.98 

-0.5 
-0.7 

-0.1 

-0-5 
-0.14 

-0-35 
-0.3 

-0.8s 
-0.85 
-1. 12 

-0.85 
-1. 17 

7-4 

9-9 
18.7 

3-3 

4-35 

9-8s 

5-35 

7.12 

14.27 

4.6 

5-95 
12. 5 

5-04 
6.6 

13-7 

-0.44 
-0.65 
-1.2 

-0.31 
-0.52 
-0.5 

-0.75 
-1. 17 
-1.77 

36.0 

17-5 

26.75 

23-05 

25-5 

-2.4 

-1.25 

-3-7 

77-15 

39-35 

58.24 

49-7 

55-43 

5-58 

2.64 

8-54 

Volumes  of  bones  (cubic  centimeters). 

Humerus 

Femur 

Tibia-fibula 

5-2 

9.1 
7-1 

2-5 

5-0 
4.0 

3-85 
7-05 

5-55 

3-1 
6-5 

5-0 

3-65 
6.94 

5-40 

-o-SS 
-0.44 
-0.40 

—0.20 

-O.II 

-0.15 

-0-75 
-0.55 
-0.55 

PART    III.  — SKELETAL    DIMENSIONS. 


Skeletons  were  prepared  of  certain  of  the  rabbits  concerned  in  this 
series  of  experiments,  and  uj)on  these  several  series  of  measurements  were 
made.     The  most  complete  scries  arc  recorded  in  tables  29  and  30. 

In  one  case  (cross  i,  table  3)  the  skeletons  of  both  parents  were  pre- 
served, as  well  as  that  of  one  of  the  fully-grown  young,  viz,  d"  248.  The 
measurements  of  this  animal  (recorded  in  table  29)  are  approximately 
intermediate  between  those  of  his  respective  parents.  They  include  7 
different  skull  measurements  and  7  of  other  parts,  chiefly  bones  of  the 
appendages. 

The  skull  of  the  lop-eared  rabbit  is  relatively  much  longer  and  more 
slender  than  that  of  short-eared  rabbits.  (See  plate  4.)  The  proportions 
of  half-blood  lops  (like  their  absolute  dimensions)  are  intermediate,  cor- 
responding closely  with  the  mid-parental  or  mean  of  the  parents  in  this 
respect.     (Sec  table  29,  ratios.) 

The  limb  bones  are  shorter  in  proportion  to  the  length  of  the  innominate 
bone  in  lop-eared  than  in  short-eared  rabbits.  In  this  j)articular  also 
part-blood  lops  are  intermediate.     (See  tables  29  and  30,  ratios.) 

In  the  case  of  the  rabbit  <?  248,  table  29,  the  deviation  from  the  raid- 
parental  measurements  or  proportions  in  no  case  equals  one-fifth  of  the 
difference  between  the  parents;  in  most  instances  it  is  much  less.  In  this 
animal  the  inheritance  of  skeletal  dimensions  and  proportions  is  unmis- 
takably blending. 

Measurements  of  another  half-blood  lop  (9  167)  are  recorded  in  table 
30.  The  mother's  skeleton  was  not  preserved.  She  was  a  short-eared 
rabbit  similar  to  ^4^.  If,  then,  the  inheritance  was  blending  also  in  the 
case  of  9  167,  her  measurements  and  j)roportions  should  resemble  those 
of  <?  248,  table  29.     This,  it  will  be  observed,  is  the  case. 

Measurements  of  a  third  half-blood  lop  (9  178)  are  recorded  in  table 
30.  The  father  of  this  rabbit  also  was  the  old  male  lop,  table  30.  The 
mother  was  the  Belgian  hare  (9431,  table  ia).  In  size  and  proportions 
of  parts  the  Belgian  hare  occupied  an  intermediate  position  between  the 
lop-eared  and  the  small  short-eared  races  used.  Accordingly  it  is  not 
surprising  to  find  that  the  half-blood  daughter  (9  178)  deviates  from  the 
other  half-blood  lops  examined,  both  in  absolute  measurements  and  in 
proportions  of  parts,  being  more  like  lop-eared  rabbits  than  they  are. 

41 


42 


INHERITANCE    IN    RABBITS 


A  sister  of  9  178,  viz,  $  175  (table  5),  had  a  son  ((^492)  by  the  lop- 
eared  male  179.  This  last-named  rabbit  was  a  son  of  the  old  female  lop 
whose  skeletal  measurements  are  recorded  in  table  29  and  of  the  old  male 
lop  whose  skull  measurements  are  recorded  in  table  30.  His  own  skele- 
ton was  not  preserved,  nor  was  the  skeleton  of  9  175  preserved,  but  if 
each  was  in  skeletal  character  the  mean  of  its  parents,  and  if  their  son 
((5*492)  was  intermediate  between  them  in  character,  we  should  expect 
his  measurements  to  resemble  the  dimensions  entered  in  column  4  of  table 
30.  A  comparison  of  this  column  with  the  next  one  shows  that  such  was 
the  case. 

Table  29.  —  Bone  measurements  of  S  248  and  of  his  parents. 


Measurement. 


1.  Total  length  of  skull 

2.  Length,  incisor  to, palate 

inclusive 

3.  Length,     occipital      to 

palate  inclusive 

4.  Width,  anterior  to  orbit . 

5.  Width,  posterior  to  orbit 

6.  Width,  at  auditory  buUas 

7.  Length  jugal  arch   .... 

8.  Length  lower  jaw 

9.  Length  femur 

10.  Length  tibia 

11.  Length  humerus 

12.  Length  ulna   

13.  Length  radius 

14.  Length  innominate .... 

Ratio: 

4  to  I 

5  to  I 

1 1  to  I 

13  to  I 

II  to   14 


Old 

female 

lop 

(mother). 


mm. 
111.3 

Si-8 

73-0 
46.8 

25-3 
39-8 
43-9 
88.3 

95-8 
116. 4 

77-3 

88.5 

74.0 

103.0 


0.420 

•233 
.694 
.665 

•75° 


0^45 
(father). 


mm. 
86.2 

36. 

57-8 
40.8 
26.9 
34-0 
37-2 
68.2 

85- 
98.1 

67-3 
74-S 
62.5 
81.7 


0.473 
■315 
.781 

•725 
.824 


Differ- 
ence 
between 
parents. 


mm. 

25-1 

15.8 

15.2 
6.0 
1.6 
5-8 
9-7 
20.1 
10.8 

18.3 
10. o 
14.0 

"•5 
21.3 


0-053 
.082 
.087 
.060 
.074 


Mean  of 
parents. 


mm. 
98.7 

43-9 

65-4 
43-8 
26.1 

369 
42.1 
78.2 
90.2 
107.2 

72-3 
81. 5 
68.2 

923 


0.446 
•  274 
•737 
•695 

.787 


c?248 
(son). 


mm. 
98.1 

430 

63.0 

44-3 
25.8 

36-7 
42.8 
80.9 
90.2 
108.7 
71.7 
82.7 
67.8 
923 


0.451 
.262 

•731 
.691 

•777 


Devia- 
tion 
from 

mean. 


mm. 
-0.6 

-0.9 

-2.4 
+  O.S 
-0.3 
-0.2 
-f-0.7 
+  2.7 
o 

+  1-5 
-0.6 
+  1.2 
-0.4 
o 


+  0.005 

—  .012 

—  .006 

—  .004 

—  .010 


Per  cent 

of  dif. 

between 

parents. 


2.6 

5^7 

15.8 

8^3 
18.8 

3^4 
7.2 

13^4 
o 

8.2 
6.0 
8.6 

3^5 
o 


9.4 

14.6 

6.9 

6.6 

i3^S 


A  brother  of  9  178,  viz,  c?  176  (table  5),  was  mated  with  the  old  female 
lop  and  had  young  which  are  described  in  table  6.  One  of  these  was  the 
three-quarter-blood  lop  9  504.  Certain  of  her  skeletal  measurements  are 
recorded  in  the  last  column  but  one  of  table  30.  Her  skull  unfortunately 
was  accidentally  destroyed  in  preparation.  If  c?  176  had  skeletal  measure- 
ments hke  those  of  his  sister  (9  178)  we  should  expect  the  daughter  (9  504) 
to  approximate  the  dimensions  entered  in  column  6,  table  30,  which  is 
the  case.  In  fact,  however,  c?  176  had  ears  less  long  than  those  of  his 
sister,  and  it  is  probable  that  his  skeletal  dimensions  also  were  less,  which 
would  account  for  the  fact  that  9  504  falls  somewhat  below  the  skeletal 
dimensions  given  in  table  30,  column  6. 


SKELETAL    DIMENSIONS 


43 


The  measurements  made  upon  the  2  tliree-ciuarter-blood  lops  (.^492 
and  9  504)  indicate  that  in  their  production,  as  in  that  of  half-loj^s,  the 
inheritance  of  skeletal  dimensions  is  blending. 

It  would  be  premature  to  conclude  that  such  is  the  case  in  all  mammals. 
Farrabee  (:o5)  has  shown  that  in  man  hypoph)  langia  (2-jointcd  fingers  and 
toes)  is  associated  with  an  abnormal  shortness  of  the  arms,  legs,  and  trunk. 
It  would  seem  that  all  the  skeletal  parts  are  abnormally  shortened.  In- 
heritance in  this  case  is  clearly  not  blending,  but  alternative.  Some  dis- 
continuous alteration  has  evidently  occurred  in  the  growth-character  of 
cells  that  form  the  skeleton,  just  as  in  the  activities  of  the  follicle  cells  in 
long-haired  mammals  (see  Castle  and  Forbes,  :  06).  It  would  be  of  inter- 
est to  know  whether  such  is  the  case  also  in  bantam  fowls  and  Shetland 
ponies. 

Table  30.  —  Bone  measurements  0/  rabbits. 


Measurement. 


1.  Total  length  of  skull  .  . 

2.  Length,  incisors  to  pal- 

ate inclusive 

3.  Length,     occipital     to 

palate  inclusive 

4.  Width,  anterior  to  orbit. 

5.  Width,  posterior  to  orbit 

6.  Width,  at  auditory  bullae 

7.  Length  jugal  arch    .  .  .  . 

8.  Length  lower  jaw 

9.  Length  femur   

10.  Length  tibia    

11.  Length  humerus 

12.  Length  ulna    

13.  Length  radius 

14.  Length  innominate    . .  . 

Ratio: 

4  to  I 

5  to  I 

1 1  to  I 

13  to  I 

II   to   14 


Old 
d'lop. 


mm. 
103-7 

48.0 

67.4 

46.3 
27.4 

364 

45-9 


0.446 
.264 


Half- 
blood 
lop 
9178. 


mm, 

104.5 

48.4 

66.5 

45-4 
27.1 

370 

87.6 
99.0 

79.6 
90.2 

75-9 
100 


0.43.S 
.259 
.761 
.726 
.796 


Expected    Three- 
mean  of    quarter- 
ed 1 79    !    Wood 
and  lop 

(^178.       d'49«- 


mm. 
106.0 

49.1 

68.3 
46.6 
26.7 
38.1 


0.440 
.252 


mm. 
107-5 
49.0 

68.6 
47-4 
37-5 
39-6 
46.0 
86.4 

100. 

1 19.6 
78.9 
91.8 
76.7 
97-4 


0.441 
.248 
•734 
-713 
.810 


Mean  of     Three- 
old  9  lop  quarter- 


(table 

29) and 

9178. 


mm. 


87.9 
97-4 

78.4 
893 

74-9 
loi.s 


blood 

lop 

9304- 


0.772 


84.0 

95-7 
109.0 

75-5 
88.0 

73-5 
97-3 


Half- 
blood 
lop 
9167. 


0-775 


98.5 

45-3 

63.4 
43-4 
25.1 

35-9 
4«-7 

89.1 
105. 1 
72.6 
81.7 
68.0 
90s 


0.443 

•737 
.690 

.80a 


Aside,  however,  from  such  unusual  cases,  it  seems  probable  that  skel- 
etal dimensions,  and  so  proportions  of  skeletal  parts,  behave  in  general 
as  blending  characters.  The  linear  dimensions  of  the  skeletal  parts  of 
an  individual  approximate  closely  the  mid-parental  dimensions. 

Volume  and  weight  magnitudes,  however,  follow  a  dilTerent  law,  whicli 
has  not  yet  been  clearly  made  out.  It  is  jdain  (hat  they  are  less  than  the 
mid-parental  magnitude.     (See  Part  II.) 


44  INHERITANCE   IN   RABBITS 

It  is  probable  that  in  plants,  as  well  as  in  animals,  linear  dimensions 
are  in  general  blending  in  their  inheritance.  In  regard  to  the  height  of 
maize,  Lock  (:o6,  p.  130)  says: 

Some  of  the  strains  which  were  made  use  of  were  uniformly  much  taller  than  others.  In  Fi 
the  height  of  the  cross-breds  between  such  strains  was  obviously  intermediate.  In  a  number 
of  cases  the  cross  was  made  between  Fi  plants  and  the  shorter  of  the  parental  types.  The 
offspring  of  this  cross  showed  no  such  segregation  into  short  and  intermediate  plants  as  was 
to  be  expected  if  Mendel's  law  held  good.  On  the  contrary,  the  plants  produced  were  re- 
markably uniform  in  height. 

This  account  agrees  precisely  with  our  observations  upon  the  inheri- 
tance of  linear  dimensions  in  rabbits. 

The  obviously  blending  inheritance  of  height  in  this  case  does  not  con- 
tradict the  known  Mendelian  behavior  of  the  growth-habit  in  such  plants 
as  the  sweet  pea,  where  Bateson  (confirming  Mendel)  has  shown  dwarf ness 
to  be  alternative  with  tallness.  Dwarfness  is  plainly  such  a  discontinuous 
variation  in  plants  as  is  hypophylangia  in  man,  and  its  inheritance  is  quite 
different  from  that  of  ordinary  variations  in  height.  The  former  is  a 
discontinuous  variation,  Mendehan  in  its  inheritance;  the  latter  belongs 
to  a  series  of  continuous  variations,  and  is  blending  in  its  inheritance. 
In  a  dwarf  plant  the  internodes  are  shortened  throughout  the  entire 
plant,  just  as  in  a  case  of  hypophylangia  there  is  a  general  shortening 
of  the  skeletal  parts. 


PART  IV.  — COLOR. 


COLOR  VARIATION  IN  RELATION  TO  COLOR  FACTORS. 

A  preliminary  discussion  of  color  variation  in  the  rabbit  was  made  by 
Castle  (:07a).  Since  that  paper  was  written  several  obscure  points  have 
been  cleared  up.  In  the  light  of  our  present  knowledge  an  attempt  will 
be  made  to  describe,  in  terms  as  simple  as  possible,  the  color  varieties 
of  rabbits  and  the  mode  of  their  production. 

The  gray  pigmentation,  common  to  wild  rabbits,  is  comj)lex  in  its  nature, 
and  all  other  color  varieties  are  relatively  simjjler.  The  gray  coat  results 
from  the  joint  action  of  several  independent  color  factors;  all  other  types 
of  pigmentation  result  from  a  weakening  or  entire  loss  of  one  or  other  of 
the  several  factors  concerned  in  producing  a  gray  coat.  In  other  words, 
color  variation  in  the  rabbit  is  wholly  retrogressive.  We  are  able  to  recog- 
nize the  existence  in  the  gray  coat  of  the  rabbit  of  8  independent  factors. 
To  assume  the  existence  of  so  many  factors  will  probably  seem  to  some 
absurd;  at  first  it  seemed  so  to  us;  but  we  have  been  forced  step  by  step 
to  the  assumption  that  they  exist  as  the  simplest  way  of  explaining  the 
observed  facts. 

The  factor  hypothesis  was  first  introduced  by  Cudnot  (:o3)  to  explain 
the  latent  transmission  of  pigment  characters  through  albinos;  it  was  devel- 
oped independently  by  Tschermak  (:o3)  to  explain  similar  phenomena 
(kryptomerism,  the  existence  of  hidden  factors)  in  beans;  and  has  been 
further  extended  by  Bateson  (:o6)  and  his  associates. 

The  8  color  factors  which  are  recognizable  in  the  case  of  tlie  gray  rab- 
bit, and  the  symbols  which  we  shall  use  to  designate  them,  are  as  follows: 

Symbol   XT.    A  common  color  factor  necessary  to  the  production  of  all 
pigment,  wanting  only  in  alljinos. 
B.    A  factor  for  black,  some  substance  which  acting  upon  C 
produces  black  pigmentation. 
~Br.    A  factor  for  brown,  some  substance  which  acting  upon  C 

produces  a  chocolate-brown  pigmentation. 
— Y.    A  factor  for  yellow,  some  substance  which  acting  upon  C 
produces  yellow  pigmentation. 
I.    An  intensity  factor,  which  determines  whether  the  pigmenta- 
tion shall    be  intense  (as  in  black  and   in   yellow),  dilute 
(as  in  blue  and  in  cream),  or  of  some  inttrmcdiate  degree 
of  intensity. 
A.    A  pattern  factor  which  causes  the  black  or  brown  pigments 
to   be   excluded   from   certain    portions   of   the   individual 

45 


46  INHERITANCE    IN   RABBITS 

hairs,  where  yellow  then  shows.  A  "ticked"  gray  coat 
results.  When  this  factor  is  present  the  under  surfaces 
of  the  rabbit  (tail,  belly)  are  unpigmented  (white).  The 
symbol,  A,  stands  for  agouti,  this  factor  having  first  been 
demonstrated  in  the  "agouti"  guinea-pig.  (See  Castle, 
:o7.) 

U.  A  factor  for  uniformity  of  pigmentation  (in  distinction  from 
spotting  with  white,  S). 

E.  A  factor  governing  the  extension  of  black  and  of  brown  pig- 
mentation, but  not  of  yellow.  When  most  restricted  in 
distribution  the  black  or  brown  pigments  are  found  in  the 
eye  and  in  the  skin  of  the  extremities  only,  but  not  in  the 
hair,  when  more  extended  they  occur  also  in  the  hair 
generally. 

DEVELOPMENT   OF   THE   FACTOR  HYPOTHESIS. 

Scientific  hypotheses,  to  be  of  service,  should  be  as  simple  as  possible. 
Therefore  no  unnecessary  assumptions  should  be  made.  To  assume 
the  existence  in  gray  rabbits  of  eight  independent  color  factors  requires 
justification. 

The  General  Color  Factor,  C. 

The  existence  of  a  color  factor  (C)  was  first  suggested  by  Cuenot  (:  03) 
to  explain  how  it  is  that  albinos  transmit  in  crosses  the  particular  colors 
which  were  borne  by  their  pigmented  ancestors.  This  common  color 
factor  being  acted  upon  by  specific  substances  (perhaps  color  enzymes) 
produces  specific  pigments,  such  as  black,  brown,  or  yellow.  No  hypoth- 
esis simpler  than  this  has  been  suggested,  nor  any  other  which  adequately 
accounts  for  the  observed  facts. 

The  Specific  Pigment  Factors,  B,  Br,  and  Y. 

The  existence  of  separate  factors  for  black  and  for  brown  pigmentation 
is  shown  beyond  question  by  the  results  of  crossing  black  with  brown 
varieties,  in  guinea-pigs  and  in  mice.  In  rabbits  a  brown  variety  is  not 
known  to  us  personally,  though  we  have  been  informed  that  such  a  vari- 
ety exists  in  continental  Europe. 

The  existence  of  a  separate  factor  (Y)  for  yellow  pigmentation  can 
scarcely  be  questioned,  in  view  of  the  fact  that  the  yellow  pigmentation 
is  as  regards  distribution  quite  independent  of  both  black  and  brown, 
remaining  extended  throughout  the  fur  when  they  are  restricted  to  the 
eyes  and  the  skin  of  the  extremities. 

The  Intensity  Factor,  I  or  D. 

The  existence  of  an  intensity  factor  was  first  announced  by  Bateson 
(:  06)  as  having  been  demonstrated  by  Miss  Durham  in  the  case  of  mice. 

For  guinea-pigs  and  rabbits  we  are  able  to  confirm  completely  Miss 
Durham's  discovery.     Since  dilution  or  concentration  of  pigment  is  a 


COLOR  47 

property  transferable  from  one  pigment  (as  black)  to  another  (as  yellow), 
it  is  evidently  due  either  to  some  modification  in  C,  or  else  to  an  inde[)en- 
dent  factor.  But  it  can  not  be  due  to  C,  since  it  is  transmissible  through 
an  albino,  which  by  hyj)Olhesis  lacks  C.  We  are  forced  to  conclude  that 
it  is  transmitted  through  some  independent  factor,  which  we  shall  desig- 
nate I,  intensity;  it  is  alternative  with  D,  a  state  of  dilution  (as  in  the  blue 
modification  of  black,  or  the  cream  modification  of  yellow). 

The  Factor  for  a  Pigment  Pattern  of  the  Individual  Hair,  A. 

Evidence  for  the  existence  of  a  factor  (A)  governing  the  |>igment  i)at- 
tern  of  the  individual  hair  has  been  jjresented  elsewhere  (Castle,  :07a). 
It  was  first  recognized  in  the  case  of  the  guinea-pig  (Castle,  :o6)  as  an 
essential  factor  of  the  "agouti"  coat,  indeed  as  the  only  feature  which 
differentiates  the  agouti  variety  from  black.  Hence  the  symbol  A  (agouti) 
was  adopted  to  designate  it.  Cuenot  (104)  employed  the  symbol  G  to 
designate  in  mice  the  agouti  or  gray  coat,  and  designated  black  by  a  differ- 
ent symbol,  but  he  failed  to  recognize  that  gray  is  simply  black  plus  a 
second  factor.  Hence  his  G  equals  B  (black)  plus  A.  Hurst  has  inde- 
pendently discovered  the  existence  of  the  A  factor  in  rabbits  (Proceedings 
Seventh  International  Zoological  Congress,  unpublished).  In  the  guinea- 
pig,  a  new  color  variety,  cinnamon-agouti,  has  been  dehberately  produced 
through  the  agency  of  the  independent  factor  A.     (See  Caistle,  :  08.) 

The  Factor  for  Uniformity  of  Pigmentation,  U,  or  Spotting  with  White,  S. 

The  factor  U  (uniformity  of  pigmentation)  is  alternative  with  spotting 
with  white,  S.  Its  existence  was  first  established  by  Cudnot  (104).  Like 
I,  the  intensity  factor,  it  may  be  regarded  as  a  modifier  of  C,  though  not 
identical  with  it;  for  U  and  S  are  transmissible  through  albinos,  which 
themselves  have  no  pigmentation  and  which  by  h}pothesis  lack  the  fac- 
tor C.  U  is  also  demonstrably  independent  of  any  particular  color,  for 
spotting  with  white  is  transferable  in  crosses  from  one  color  variety  to 
another,  as,  for  example,  from  black  to  yellow. 

The  Factor  for  Extended  Distribution  of  Black  or  Brown,  E,  Alternative 

with  R  (Restricted  Distribution). 

The  assumed  factor  E  is  a  modifier  of  black  and  brown,  but  not  of 
yellow  pigmentation.  It  is  alternative  to  R,  a  restricted  distribution  of  black 
and  of  brown  jMgments,  in  which  distribution  they  are  conlined  to  the  eyes 
and  to  the  skin  of  the  extremities.  The  distribution  of  yellow  pigment 
(Y)  is  wholly  unaffected  by  this  factor.  When  black  and  brown  are 
restricted,  yellow  remains  as  the  principal  or  even  as  the  exclusive  pig- 
mentation of  the  hair  (yellow  varieties). 

That  E  really  exists  as  an  independent  factor,  and  not  as  a  condition 
merely  of  black  or  of  brown,  is  shown  by  the  following  ex|X'riment.  If 
one  crosses  a  brown   ("chocolate")   guinea-pig  with   an  ordinary  yellow 


48  INHERITANCE   IN   RABBITS 

one  (black-eyed),  the  young  are  black  pigmented,  but  in  F^  4  varieties 
are  obtained,  viz,  black,  brown,  black-eyed  yellow,  and  brown-eyed 
yellow.  The  case,  at  first  thought  puzzUng,  is  entirely  plain  if  we  con- 
sider the  distribution  independent  of  the  kind  of  pigment.  In  the  original 
cross  extended  brown  was  combined  with  restricted  black.  Extension 
dominated  restriction,  and  black  dominated  brown,  but  in  Fj  black  and 
brown  each  occurred  both  in  the  extended  and  in  the  restricted  condi- 
tion. Plainly  the  case  is  one  of  MendeUan  dihybridism,  in  which  two 
independent  pairs  of  alternative  characters  are  concerned. 

The  extension  factor  (E)  may  be  replaced,  not  merely  by  the  extreme 
condition  (R)  in  which  black  and  brown  pigment  are  absent  from  the  fur, 
but  also  by  conditions  of  restriction  less  extreme,  in  which  spots  of  black 
(or  brown)  occur  on  a  background  of  yellow.  Such  intermediate  con- 
ditions (E',  E",  etc.)  are  heritable,  and  are  alternative  with  E  and  R, 
respectively.  In  some  of  these  intermediate  conditions  the  spots  are  of 
large  size  and  sharply  limited,  in  others  the  spots  are  numerous  and  small. 
Each  condition  has  a  tendency  to  breed  true,  i.  e.,  is  alternative  to  other 
conditions  of  E. 

INTERRELATIONS   OF   FACTORS   E   AND   U. 

Spotting  with  black  or  brown  on  a  yellow  background  is  independent  of 
spotting  with  white,  though  the  two  may  coexist.  The  one  is  due  to  a 
modification  of  E,  the  other  to  a  modification  of  U.  When  E  and  U  are 
both  unmodified  the  animal  is  of  course  black  (or  brown)  pigmented  all  over. 
When  U  alone  is  modified  (and  occurs  in  condition  S) ,  the  animal  is  black 
(or  brown)  but  spotted  with  white.  When  E  is  modified  (to  E'  or  E") 
but  U  is  unmodified,  the  animal  is  spotted  with  black  (or  brown)  on  a 
yellow  background,  but  is  devoid  of  white.  When  both  E  and  U  are 
modified  (to  E'  or  E"  and  to  S,  respectively)  the  animal  bears  two  differ- 
ent sorts  of  colored  spots  on  a  white  background.  The  spots  are  either 
black  and  vellow  or  brown  and  vellow,  and  constitute  with  the  white 
background  on  which  they  lie  the  so-called  "tricolor"  condition,  well 
known  in  the  case  of  guinea-pigs,  dogs,  cats,  and  mice. 

It  is  a  singular  fact  that  spots  of  black  and  of  brown  do  not  occur  on 
the  same  animal,  so  a  4-colored  condition  is  never  attained.  The  reason 
for  this  is  apparent,  if  the  hypothesis  stated  in  this  paper  is  correct.  The 
distribution  of  black  and  of  brown  is  controlled  by  the  same  factors,  E  and 
S,  so  that  when  black  and  brown  are  present  together,  their  distribution 
is  the  same,  and  black  because  of  its  greater  opacity  covers  up  the  brown. 

The  " black-and-tan "  dog  is,  we  believe,  an  apparent,  not  a  real,  excep- 
tion to  this  generahzation;  for  the  "tan"  is  not  a  chocolate-brown  pigment 
such  as  is  found  in  the  brown  water-spaniel,  but  merely  a  yellow  pigment. 
The  black-and-tan  dog  is  not  a  spotted  dog,  but  is  a  black  dog  plus  a  color- 
pattern,   similar  to   the   agouti-pattern  of  guinea-pigs  and   rabbits.     In 


COLOR  49 

this  pattern  black  is  largely  excluded  from  the  lower  surfaces  and  from 
a  spot  over  each  eye,  where  yellow  then  shows.  The  correctness  of  this 
hypothesis  is  shown  by  the  existence  of  this  same  i)attern  anmng  brown- 
pigmenled  dogs.  The  brown-and-tan  has  chocolate-brown  i)igment  above 
and  tan  (yellow)  below,  as  well  as  a  spot  over  each  eye.  It  bears  the  same 
relation  to  self-brown  that  black-and-tan  docs  to  self-black.  On  this 
interpretation  brown-and-tan  is  brown  plus  |)attern,  and  black-and-tan 
is  black  plus  pattern.  If,  then,  brown-and-tan  is  crossed  with  self-black, 
black-and-tan  offspring  should  result  in  Fj,  and  in  F,  there  should  be 
obtained  black-and-tan,  brown-and-tan,  self-black,  and  self-brown,  in 
the  proportions  9:3:3:  i.     The  experiment  is  commended  to  dog  breeders. 

INTERRELATIONS   OF  FACTORS  B,  Br,  AND   Y. 

Returning,  after  this  digression,  to  a  consideration  of  the  interrelations 
of  the  three  pigments,  black,  brown,  and  yellow,  the  fact  seems  clearly 
established  that  black  and  brown  are  closely  related  but  alternative  con- 
ditions dependent  for  their  distribution  upon  two  factors,  which  we  may 
designate  E  and  S,  whereas  yellow  is  dependent  for  its  distribution  solely 
upon  one  of  these  two  factors,  S.  It  would  seem  probable,  therefore, 
that  in  the  genesis  of  the  hair  jjigments,  yellow  is  a  first  product  of  the 
interaction  of  C  and  Y,  which  may  or  may  not  be  further  modified  to  pro- 
duce brown  or  black,  depending  upon  whether  certain  other  factors  (B  and 
Br)  are  or  are  not  present.  The  amount  and  distribution  of  the  yellow 
pigment  produced  is  conditioned  by  a  factor  which  may  assume  phases 
U,  S,  S',  etc.  The  amount  of  the  yellow  pigment  which  is  converted  into 
black  or  brown  and  its  distribution  is  conditioned  by  another  factor  which 
may  assume  phases  E,  E',  etc.,  to  R. 

Gametic  Structure  and  Variation. 

A  diagram  Hke  those  employed  by  the  organic  chemist  may  help  to  show 
the  relationships  to  each  other  of  these  8  assumed  pigment  factors. 

C,  the  general  color  factor,  is  indisj)ensable  y  B 

to  the  manifestation  of  any  of  the  others.  All 
the  others  may  be  represented  as  linked 
directly  or  indirectly  with  it.  E,  however,  is 
a  modifier  of  B  and  Br  alone,  and  is  there- 
fore joined  with  them  alone  in  the  diagram;  and  since  B  and  Br  are 
assumed  to  act  only  after  Y  has  acted,  they  are  represented  as  joined 
with  it. 

Homozygous  gray  rabbits,  wild  ones  for  exam|)le,  possess  and  transmit 
all  these  8  factors  in  each  of  their  gametes.  The  diagram,  therefore, 
expresses  their  gametic  composition.  A  homozygous  black  rabbit  lacks, 
of  all  these  8  factors,  A  alone.  A  yellow  rabbit  has  R  (restricted)  in  place 
of  E  (extendcfl  black  or  brown),  but  otherwisi-  is  like-  the  gray,  or  else  the 


.\ C Y  E 


50  INHERITANCE    IN    RABBITS 

black  rabbit.  Those  with  A  and  those  without  A  are,  however,  visibly 
different. 

Theoretically,  if  each  factor  is  capable  of  independent  variation,  256 
different  gametic  combinations  should  be  possible.  In  reahty  we  are 
acquainted  with  18  visibly  different  color  varieties,  and  we  have  evidence 
that  48  different  gametic  combinations  are  capable  of  realization.  This 
leaves  still  a  wide  discrepancy  between  theoretical  and  known,  and  leads 
to  the  conclusion  either  that  many  as  yet  unknown  mutations  are  possible 
in  the  rabbit,  or  that  couplings  may  exist  among  these  factors  which  pre- 
vent their  independent  action. 

We  have  evidence  of  independent  variation  on  the  part  of  the  factors 

A,  C,  I,  U,  and  E,  each  of  which  has  in  one  case  or  another  either  been 
lost  or  been  replaced  by  the  alternative  condition  already  described;    but 

B,  Br,  and  Y  are  unvariable;  at  least  we  have  not  ourselves  seen  evidence 
among  rabbits  of  independent  variation  on  the  part  of  these  factors. 
There  can  be  no  question,  however,  that  both  in  the  guinea-pig  and  in 
the  mouse  such  variation  has  occurred,  resulting  in  the  complete  loss  of 
B  from  the  gamete,  and  it  is  possible,  as  elsewhere  stated,  that  such  a 
change  has  already  occurred  among  European  rabbits.  Supposing,  how- 
ever, that  B,  Br,  and  Y  are  all  constant  constituents  of  the  rabbit  gamete 
and  that  each  of  the  five  others  may  be  either  present  or  absent,  the  num- 
ber of  different  gametic  combinations  theoretically  possible  becomes  32. 
We  have  reason  to  believe  that  this  entire  assortment  is  produced  and 
that  16  other  ones  also  occur  owing  to  a  second  and  different  sort  of  vari- 
ation in  factor  C. 

Gametic  and  Zygotic  Formul-e. 

The  diagram  given  on  page  49  was  intended  to  express  the  known  aggre- 
gate of  independent  factors  which  a  pure  gray  rabbit  transmits  in  each  of 
its  reproductive  cells  (gametes).  In  producing  a  new  individual  each  re- 
productive cell  must  unite  with  another  reproductive  cell,  the  two  together 
forming  a  zygote.  An  individual  resulting  from  the  union  of  two  gam- 
etes of  hke  constitution  will  be  double  as  regards  each  hereditary  factor. 
It  is  known  as  a  homozygote  (Bateson).  This  double  condition  we  might 
express  by  a  subscript  2  following  the  symbol  for  each  factor  indicated. 
We  should  then  have  a  zygotic  formula  for  the  individual. 

But  it  sometimes  happens  that  a  gamete  unites  with  another  gamete 
having  a  composition  slightly  different  from  its  own  —  one  which,  for 
example,  lacks  one  or  more  factors  found  in  itself.  The  zygote  produced 
is  then  a  heterozygote  and  will  be  double  as  regards  certain  factors,  but 
single  as  regards  others.  But  in  sexual  reproduction,  as  is  well  known, 
there  is  a  return  from  the  double  to  the  single  condition.  So  that  when 
a  heterozygous  individual  attains  sexual  maturity,  it  forms  gametes  each 
of  which  contains  the  factor  double  in  the  zygote,  but  as  regards  those 
which  were  single  in  the  zygote,  half  the  time  they  will  be  present,  half 


COLOR  51 

the  time  absent  from  the  f^amete  (or  if  not  absent,  then  represented  by 
an  ahernativc  condition).  This  is  simply  another  way  of  stating  the  funda- 
mental ^Slendelian  i)rinciple  that  heterozygotes  do  not  breed  true,  but 
form  at  least  two  difTerent  kinds  of  rej)roductive  cells. 

The  breeder  has  to  deal  ahvavs  with  individuals,  and  onlv  inrlirecth 
with  gametes.  Therefore  zygotic  formulae  are  to  him  quite  as  im[K)rtant 
as  gametic  formula?.  Accordingly  in  what  follows  we  shall  endeavor  to 
give  the  zygotic  formula  of  each  variety  described.  Its  breeding  capac- 
ity may  quickly  be  inferred  from  an  insi)ection  of  its  zygotic  formula.  Kach 
factor  which  is  double  in  the  zygote  will  be  rejiresented  in  every  gamete 
formed,  each  factor  which  is  single  in  the  zygote  will  be  present  in  only 
half  the  gametes  formed,  or  will  be  represented  b\'  the  alternative  (reces- 
sive) condition  expressed  in  the  zygotic  formula  by  a  symbol  in  paren- 
thesis. 

The  zygotic  formula  of  a  gray  ra])bit  which  breeds  true  (an  ordinary 
wild  one,  for  example)  is  BoBroEjAX^LU^Yz,  and  the  interrelations  of 
these  factors,  as  at  present  understood,  may  be  expressed  in  a  diagram. 

A, C, Y,  E2 

Other  gray  rabbits  arc  single  (or  heterozygous)  as  regards  one  or  more 
of  the  factors  enumerated  in  this  formula,  though  none  of  them  lacks 
altogether  any  one  of  these  8  factors.  When  a  factor  drops  out  altogether 
a  new  color  variety  is  produced.  New  color  varieties  have  undoubtedly 
originated  in  this  way  in  the  past,  and  are  still  doing  so  at  the  present 
time.  A  maturation  division  in  which  the  two  components  of  a  double 
factor  should  fail  to  separate  (as  they  do  normally)  might  be  the  starting- 
point  of  a  new  color  variety,  since  it  would  result  in  the  ])roduction  of  a 
gamete  which  lacked  a  particular  factor.  Abnormal  maturation  diu- 
sions,  therefore,  may  be  the  immediate  cause  of  color  variations. 

COLOR   VARIETIES   OF   THE   RABBIT. 

It  is  impossible  to  make  a  scientific  classification  of  the  color  varieties 
of  the  rabbit  without  discarding  or  modifying  some  of  the  names  now 
in  use;  for  many  of  these  names  are  either  without  significance  or  are 
misleading.  From  a  perusal  of  the  literature  of  the  rabbit-fancy,  we  arc 
unable  to  decide  what  certain  named  varieties  are,  and  it  is  more  than 
likely  that  we  are  not  acquainted  at  first-hand  with  many  varieties  known 
to  the  fancy  in  Europe.  All  such  cases  must  necessarily  be  omitted,  for 
the  present,  from  our  classification. 

For  convenience  we  may  recognize  4  general  color  types,  viz,  (i)  gray, 
(2)  black,   (3)   yellow,  and   (4)  white.     Each  of  the  pigmented  varieties 


52  INHERITANCE    IN   RABBITS 

(gray,  black,  and  yellow)  may  have  either  intense  or  dilute  pigmentation 
(disregarding  intermediate  shades,  which,  however,  exist  and  are  heri- 
table) .  Further,  each  may  either  have  uniform  pigmentation  or  be  spotted 
with  white  (disregarding  differences  in  the  fineness  of  the  spotting,  which, 
however,  exist  and  are  heritable).  Further,  the  yellow  may  have  either 
pigmented  or  white  under  surfaces.  Even  with  categories  so  inclusive 
as  these,  the  number  of  visibly  different  pigmented  varieties  rises  to  i6, 
and  since  albinos  may  either  have  or  not  have  pigmented  extremities,  the 
total  number  of  visibly  different  varieties  mounts  to  i8. 

There  is  every  reason  to  suppose  that  each  of  these  i8  varieties  may  be 
obtained  in  a  homozygous  condition.  Most  of  them,  indeed,  have  been 
so  obtained  in  our  experiments.  But  for  each  homozygous  condition 
there  are  possible  several  heterozygous  conditions.  An  enumeration  of 
all  these  is  unnecessary,  as  the  number  is  truly  stupendous.  With  5  inde- 
pendently variable  characters  (the  number  known  to  be  independently 
variable  in  the  rabbit)  the  number  of  different  zygotic  combinations  theoret- 
ically possible  is  243. 

We  shall  content  ourselves  with  enumerating  the  18  different  known 
gametic  combinations,  and  in  giving  examples  of  a  few  of  the  different 
zygotic  combinations. 

Gray  Type. 
(i)    Gray,  found  in  wild  rabbits;  gametic  composition  — 

A C Y  E 

I         \   / 

I  Br 

(2)  Blue-gray,  same  as  the  foregoing,  with  the  substitution  of  D  (dilute) 

for  I  (intense). 

(3)  Spotted  gray,  same  as  i,  with  the  substitution  of  S  (spotted)  for  U 

(uniform  pigmentation). 

(4)  Spotted  blue-gray,  same  as  2,  with  the  substitution  of  S  for  U. 

Black  Type. 

(5)  Black,  same  as  i  without  A,  namely, 

f       /\ 

C Y  E 

t        \/ 

(6)  Blue  (i.  e.,  dilute  black),  same  as  5,  with  the  substitution  of  D  for  I. 

(7)  Spotted  black,  same  as  5,  with  the  substitution  of  S  for  U. 

(8)  Spotted  blue,  same  as  6,  with  the  substitution  of  S  for  U. 


COLOR  53 

Yellow  Type. 

(9)  Yellow   (with  white  belly  and   tail),  same  as  i,  with  R  (restricted) 

substituted  for  K  (extended  black  or  brown  pigmentation),  namely, 

y    /\ 

A C Y  R 

1        \/ 

(10)  Cream  (/.  e.,  dilute  yellow),  same  as  9,  with  D  substituted  for  I. 

(11)  Spotted  yellow,  same  as  9,  with  S  substituted  for  U. 

(12)  Spotted  cream,  same  as  10,  with  S  substituted  for  U. 

(13)  Sooty  (yellow  with  pigmented  belly  and  tail),  same  as  9  without 

A,  or  as  5  with  R  substituted  for  E,  namely, 

I  Br 

(14)  Pale  sooty,  same  as  13,  with  D  substituted  for  I. 

(15)  Spotted  sooty,  same  as  13,  with  S  substituted  for  U. 

(16)  Spotted  pale  sooty,  same  as  14,  with  S  substituted  for  U. 

WmTE  Type. 

(17)  White  (wholly  unpigmented),  in  any  of  the  foregoing  16  varieties 

with  C  omitted. 

(18)  Himalayan  white,  a  pink-eyed  albino  variety  differing  from  17  in 

appearance,  in  having  black  pigmented  extremities  (nose,  ears, 
feet,  and  tail)  and  in  having  fur  of  a  creamy  white,  not  of  a  snowy 
white  as  in  17.  Those  with  which  we  have  exjierimented  seemed 
to  be  of  the  formula '  — 


y 


-  Y  E 

1        \./ 

That  is,  they  were  black  pigmented  rabbits  (see  5)  in  all  points  except  C.  It 
would  seem  that  we  must  assume  the  presence  of  C  in  some  form  in  an  animal 
which  like  these  does  bear  a  certain  amount  of  pigment.  Nevertheless  this  C 
is  not  the  same  as  the  C  found  in  dark-eyed  pigmented  varieties,  for  a  cross  of 
Himalayan  with  other  albinos  produces  no  dark-eyed  ofTs])ring,  and  gives  no 
increase  of  pigmentation  over  that  found  in  the  Himalayan  parent,  but  rather  a 
diminution  of  it  (see  Castle,  :o5).  If,  then,  we  assume  C  to  be  j^resent  in  the 
Himalayan,  it  must  be  in  a  greatly  modified  form,  as  compared  with  its  condi- 
tion in  dark-eyed  animals.  This  is  why  we  use  C  rather  than  C  in  the  formula. 
The  factors  E,  I,  and  U,  were  all  found  to  be  present  in  our  Himalayan  rabbits, 
but  not  A,  for  crosses  of  Himalayan  with  homozygous  gray  gave  only  gray  in 

•April,  1909.  Himalayan  ral>l)its  have  now  iK-cn  produri-d  whidi  contain  al.so  factor  .A.  They 
have  extremities  lc.<w  heavily  piRmcnted  than  ordinary  Himalayans,  and  the  tail  is  'uhxtf 
underneath,  as  in  gray  and  in  yellow  rabbits. 


54 


INHERITANCE    IN   RABBITS 


Fj,  and  in  F,  gray,  black,  and  Himalayan,  but  no  other  varieties.  Whether  it 
is  possible  to  associate  A  with  the  other  factors  found  in  a  Himalayan  rabbit 
remains  to  be  demonstrated. 

The  reader  will  naturally  expect  some  concrete  evidence  in  support  of 
the  gametic  composition  ascribed  to  the  various  color  varieties  in  the  fore- 
going enumeration.  To  a  consideration  of  this  we  may  proceed  immedi- 
ately. It  would  be  wearisome  to  describe  in  detail  all  the  experiments 
which  have  been  made  in  the  investigation  of  this  matter.  They  have 
involved  the  production  in  various  sorts  of  matings  of  some  thousands  of 
rabbits.  It  will  suffice,  we  think,  to  cite  from  our  experiments  certain 
matings  which  were  of  such  a  nature  as  to  test  the  validity  of  our  hypotheti- 
cal formulae. 

ZYGOTIC   VARIATION   WITHIN   EACH   COLOR   VARIETY. 

GRAY. 

The  formula  has  already  been  given  of  a  gamete  which  transmits  the 
coat  characters  of  a  wild  gray  rabbit.  It  contains,  as  we  have  seen,  8 
distinct  factors.  Such  a  gamete  might  be  produced  by  gray  rabbits  of 
many  different  sorts,  all  of  which  look  alike  but  breed  differently,  i.  e., 
which  have  a  different  zygotic  composition. 

(i)  The  first  sort  which  we  will  consider  is  homozygous  (double)  as  regards 
each  factor  which  enters  into  the  composition  of  the  gamete  transmitting  gray. 
Its  zygotic  formula  is  BjBr^EjAjC^IjUjYj  (compare  diagram,  p.  51).  Every 
gamete  which  it  forms  transmits,  therefore,  all  the  components  of  a  gray  coat. 
This  is  the  condition  found  in  ordinary  wild  rabbits.  One  of  our  original  stock 
of  rabbits  (9  431))  a  Belgian  hare,  was  of  this  sort.  In  a  variety  of  crosses  she 
produced  only  gray  offspring. 

Table  31.  —  Matings  and  young  of  9  431,  the  Belgian  hare. 


Mating. 

Gray 
young. 

With  (5^56,  an  albino 

5 

3 

10 

7 

With  c?8,  a  Himalayan  albino 

With  old  lop  male,  yellow 

With  0^1 76,  her  son,  gray 

Total    

25 

The  matings  with  c?  56  and  c?  8  indicate  that  she  did  not  carry  albinism  as 
a  recessive  character;  the  mating  with  the  yellow  rabbit  shows  that  she  did  not 
carry  yellow  as  a  recessive  character.  The  yellow  rabbit  in  question  was  found 
by  other  tests  to  be  heterozygous  in  the  pattern  factor  A.  Consequently  the 
Belgian  hare  was  almost  certainly  homozygous  in  that  factor;  otherwise  half 
the  young  produced  in  this  mating  should  have  been  black  instead  of  gray. 

(2)  A  second  sort  of  gray  rabbit  produces  (when  mated  with  animals  like 
itself)  gray  offspring  and  black  ones,  but  produces  none  of  other  color  varieties 
in  any  kind  of  mating.  It  differs  from  variety  i  only  in  regard  to  the  factor 
A,  in  which  it  is  heterozygous  (single).  Its  zygotic  formula  accordingly  is 
B2Br2E2AC2l2U2Y2.  In  half  its  gametes  the  A  factor  is  transmitted  along  with 
all  the  other  7  factors;  in  half  its  gametes  the  factor  A  alone  is  wanting.     Gray 


COLOR 


65 


rabbits  of  this  sort  arc  readily  produced  by  mating  a  ^ray  rabbit  with  a  black 
one.  A  ral^bit  of  this  sort  produced  in  a  slightly  different  way  wa.s  our  ^^ay 
d"  2005,  which,  when  mated  with  a  sooty  yellow  (;  1471),  (produced  a  litter  of 
3  black  and  2  gray  young;  likewise  our  gray  d"  2004,  which,  when  mated  with 
sooty  yellow  9  1491,  produced  5  black  and  5  gray  offspring  (exactly  the  exjjccted 
equality  of  blacks  and  grays).  Many  Belgian  hares  are  of  this  second  variety 
("throwing  blacks,"  as  well  as  grays,  but  not  other  colors). 

(3)  A  third  sort  of  gray  rabbit  produces  (when  mated  with  animals  like  itself) 
albino  offs|)ring  as  well  as  gray  ones,  but  none  of  other  colors.  It  is  heterozygous 
(single)  in  C,  but  otherwise  homozygous.  Its  formula  is  BJirjIvXjCKUjY,. 
We  have  at  the  present  time  some  rabbits  believed  to  be  of  this  kind,  but  they 
are  as  yet  not  fully  tested.     Hurst  (:o5)  also  describes  rabbits  of  this  variety. 

(4)  A  fourth  sort  of  gray  rabbit  produces  '  young  of  the  varieties  gray,  black, 
and  white  only.  It  is  heterozygous  (single)  in  A  and  C,  but  not  in  other  factors. 
Its  formula  is  BjBr^EjACIjUzYj.  It  is  represented  in  our  rabbits  (9  1161  and 
c?ii72)  produced  by  a  cross  between  a  white  rabbit  (of  gray  ancestry)  and  a 
heterozygous  gray  one.  Mated  w-ith  each  other  these  two  produced  17  young, 
distributed  as  follows:  10  gray,  3  black,  and  4  white  (expected  9:3:  4).  Male 
1172  was  mated  also  with  a  white  female  of  black  parentage  (9  789),  and  pro- 
duced 3  gray,  5  black,  and  2  white  offspring  (expected  1:1:2). 

Rabbits  differing  from  variety  4  only  in  the  respect  that  the  albinos  which 
they  produce  are  of  the  Himalayan  type  are  represented  in  our  gray  <i  48,  and 
9  49  and  9  50.  They  were  produced  by  the  mating  of  the  Belgian  hare  9  431 
with  a  Himalayan  (c?  8).  The  result  of  their  matings  inter  se  is  given  in  table  32. 

Table  32.  —  Matings  and  young  of  S  48,  gray. 


Matings. 

Young. 

Gray. 

Black. 

Himalayan. 

With  940.  Erav 

9 

15 

S 
5 

7 
6 

With  9  ^0.  crav 

Total' 

24 

10 

»3 

'  Expected  ratio  of  g:  3:  4,  giving  in  this  case  27:  9:  12. 

The  formula  of  such  rabbits  may  be  expressed  by  adding  the  symbol  C  to 
the  formula  as  given  for  variety  4,  viz,  B,Br,E,.'\C(C')l2^^2Y2. 

Table  33.  — Matings  and  young  0/  gray  9  175. 


Mating. 

Young. 

Gray. 

24 
II 

4 

YeUow. 

13 
S 
a 

Black. 

Withc?i76,  gray 

WithcJ'177,  gray 

With  ^"43  7  J.  gray 

Total 

0 
0 

0 

39 
3 

30 

I 

0 
0 

Expected 

(5)  A  fifth  sort  of  gray  rabbit  produces  young  of  the  varieties  gray  and  yel- 
low, but  none  of  other  colors.  It  is  heterozvgous  (single)  in  the  extension  factor 
E,  carrying  as  an  alternative  (recessive)  character  R  (restricted  distribution  of 
black  and  brown  pigments).     Its  formula  accordingly  is  B,Br,E(R).\jr,IjUjY,. 

•  When  on  this  and  subsequent  pages  the  nature  of  the  mating  is  not  spedSed  it  will  \x  under- 
stood that  the  mating  is  with  animals  of  its  own  variety  or  of  varieties  recessive  to  its  own. 


56 


INHERITANCE    IN    RABBITS 


It  is  represented  in  our  gray  c?  2071,  which,  when  mated  with  a  sooty  yellow 
female  (9  1414),  produced  4  gray  and  3  yellow  young.  This  variety  is  repre- 
sented likewise  by  9  175  and  c?  176,  borne  by  the  Belgian  hare  (9431)  in  a 
mating  with  the  old  yellow  lop  male. 

Female  175  was  not  actually  mated  with  a  black  rabbit,  but,  had  she  been 
capable  of  producing  black  offspring,  she  should  have  done  so  in  the  matings 
with  c?  177,  a  rabbit  that  did  produce  black  young.  Male  176  was  mated  with 
two  does  known  to  be  capable  of  producing  black  offspring,  with  the  result 
shown  in  table  34. 

Table  34.  —  Matings  and  young  of  S  176,  gray. 


Mating. 

Gray. 

Yellow. 

Black. 

With  old  9  lop,  sooty  .... 

Expected 

With  9178,  gray 

Expected 

II 

I 
8 
.3 

3 

I 
2 
I 

0 
0 
0 

0 

(6)  A  sixth  variety  of  gray  rabbit  produces  gray,  black,  yellow,  and  sooty 
yellow  offspring,  but  none  of  the  other  colors.  It  is  heterozygous  both  in  E 
and  in  A.  It  differs  from  variety  5  only  in  being  heterozygous  instead  of 
homozygous  in  A.  It  may  readily  be  produced  by  crossing  yellow  with  black, 
or  gray  with  sooty  yellow,  the  result  of  these  two  crosses  being  identical.  The 
formula  of  this  variety  is  B2Br2E(R)AC2l2U2Y2.  This  variety  is  represented  in 
our  gray  rabbits  (c?  177  and  9  178),  which  were  borne  by  the  Belgian  hare  (9  431) 
in  the  same  litter  as  the  rabbits  of  variety  5  already  described,  viz,  9  i7S  and 
c?  176.     Another  rabbit  of  this  variety  was  the  gray  J  505. 


Table  35.  —  Matings  and  young  of  ^  177,  gray. 


Mating. 

Gray. 

Black. 

Yellow. 

Sooty. 

With  9175,  gray  (variety  5)  . 

Expected 

With  9  178,  gray  (variety  6)  . 

Expected 

5 
3 

I 

9 

0 
0 

2 
3 

2 
I 
0 

3 

0 
0 
0 

I 

Why  rabbits  177  and  178  should  differ  in  breeding  capacity  from  their  brother 
and  sister,  176  and  175,  is  readily  explained.  The  father  was  heterozygous  as 
regards  factor  A.  To  175  and  176,  he  transmitted  it;  to  177  and  178,  he  did 
not.  But  all  four  received  this  factor  from  their  mother,  a  homozygous  gray 
rabbit  (9431).  Hence  175  and  176  were  double  in  A,  but  177  and  178  were 
single  as  regards  A.  Evidence  for  the  classification  given  of  rabbits  177  and 
178  is  shown  in  tables  35  and  36. 

(7)  A  seventh  variety  of  gray  rabbit  should  produce  young  of  the  varieties 
gray,  yellow,  and  white,  but  none  of  other  colors.  It  should  differ  from  variety  5 
in  being  heterozygous  (single)  in  C.  Its  formula  would  be  B2Br2E(R)A2Cl2U2Y2. 
We  are  unable  to  cite  an  undoubted  example  of  this  variety,  though  it  could 
probably  be  produced  by  crossing  variety  3  with  variety  5,  as  well  as  in  several 
other  ways. 

(8)  An  eighth  variety  bears  the  same  relation  to  variety  6  that  7  does  to  5. 
It  is  of  the  formula  B2Br2E(R)ACl2U2Y2,  and  it  produces  young  of  the  5  visibly 
different  classes  —  gray,  black,  yellow,  sooty  yellow,  and  white.     This  variety 


COLOR 


57 


is  represented  in  rabbits  c?  ii6oanfl  9  1 171,  born  in  the  same  litter  with  9  1161 
and  i  1 172  of  variety  4  (p.  55).  When  mated  witli  ea(  h  other  they  pnwluted  7 
gray,  i  black,  2  yellow,  i  sooty,  and  5  white  young.  ( )ther  rabbits  <>i  this  variety 
were  produced  by  the  Belgian  hare  (9  431)  in  a  mating  with  an  albino  rabbit 
(c?  56).  These  gray  ral»bits  (2  males  and  t,  females,  232  to  236),  when  mated 
inter  se  produced  17  gray,  5  black,  6  yellow,  2  sooty  yellow  and  6  white  young, 
the  expected  Mcndelian  proportions  being  27:9:9:3:  16.  When  mated  with 
albinos  these  same  gray  rabbits  produced  9  gray,  6  black,  2  yellow,  and  16 
white  young.  The  zygotic  composition  of  the  aIl»ino  mates  in  this  case  is  not 
fully  known,  so  that  the  theoretical  jjroportions  of  the  pigmented  young  can 
not  be  stated.  The  albinos  are  as  expected  approximately  half  the  total  young, 
and  all  expected  color  varieties  are  represented  except  sooty  yellow. 

Table  36.  —  Matings  and  young  of  9  178,  gray. 


Mating. 


With  6^176,  gray  (variety  5)  . 

Expected 

Withc?i77,  gray  (variety  6)  . 
With  0^505,  gray  (variety  6)' 


Total  for  last  two  matings 
Expected 


With  o'i79,  yellow 
With  c?3 19,  yellow 


Total  for  last  two  matings 
Expected 


Gray. 

Black. 

YeUow. 

Sooty. 

8 

0 

2 

0 

3 

0 

I 

0 

I 

a 

0 

0 

S 

3 

I 

6 

5 

I 

(?) 

9 

3 

3 

I 

I 

3 

3 

0 

8 

3 

4 

6 

9 

6 

7 

6 

I 

I 

I 

I 

1  Plus  3  yellow  or  sooty,  died  young. 

For  simplicity  the  young  of  all  5  gray  rabbits  are  grouped  together  in  the 
foregoing  account.  In  reality,  however,  one  of  the  males  was  heterozygous  in 
factor  U,  and  at  least  i  male  and  i  female  were  heterozygous  in  factor  I,  as  is 
shown  by  the  facts  (i)  that  several  of  the  young  produced  in  matings  with  white 
individuals  bore  spots  of  white,  the  whitest  of  all  being  belted  with  white  ("Dutch 
marked");  and  (2)  that  one  of  the  gray  offspring  of  the  gray  parents  was  of  a 
pale  blue-gray  color. 

(9)  The  gray  rabbits  just  described,  which  produced  a  blue-gray  young  one, 
in  reality  belong  to  a  ninth  variety  of  gray  rabbit,  indistinguishable  in  appear- 
ance from  the  other  eight,  but  producing  a  different  assemblage  of  young,  viz, 
dilute-colored  as  well  as  intensely  colored  young  in  each  of  the  pigmented  vari- 
eties, and  also  albinos.  The  whole  assemblage  of  visibly  different  varieties  is 
gray,  black,  yellow,  sooty  yellow-,  blue-gray,  blue,  pale  yellow,  pale  soot)'  yellow, 
and  white.  Not  all  of  these  varieties  were  obtained  directly  from  the  pair 
of  rabbits  in  question,  doubtless  because  too  small  a  number  of  young  was 
produced,  but  in  later  generations  all  were  obtained.  Thus  from  the  single 
blue-gray  individual,  when  mated  within  the  same  family,  were  obtained  blues, 
pale  sooties,  and  pale  yellows,  as  well  as  individuals  of  normal  intcnsitv.  The 
zygotic  formula  of  this  ninth  variety  of  gray  ral)bit  is  B,BrJ-"(R).\ri(n)V,Uj. 
Since  it  indicates  a  heterozygous  condition  in  4  character-units,  we  should 
expect  a  pair  of  individuals  of  this  formula  to  produce  16  different  gametic 
combinations.  Eight  of  these  are  represented  in  the  enumerated  8  classes  of 
pigmented  young.  8  others  would  occur  among  the  albinos  which  would  differ 
from  the  pigmented  classes  in  the  absence  of  C,  but  all  of  which  would  look 
alike,  though  breeding  differently  in  crosses  with  pigmented  animals. 


58 


INHERITANCE    IN    RABBITS 


(10)  The  gray  rabbit  which  produced  spotted  offspring  in  crosses  with  albinos 
was  undoubtedly  heterozygous  in  regard  to  the  factor  U,  for  experience  has  shown 
(in  agreement  with  Hurst,  105)  that  uniform  pigmentation  is  in  the  main  domi- 
nant over  spotting.  We  might  then  recognize  as  a  tenth  variety  one  of  the 
formula  B2Br2E(R)ACl2U(S)Y2. 

(11)  Had  the  rabbit  in  question  produced  pale-pigmented  as  well  as  spotted 
young  (and  such  we  have  since  derived  from  this  stock  of  rabbits),  we  should 
need  to  modify  the  formula  as  given  by  writing  1(D)  instead  of  Ij,  i.  e.,  intense 
(dilute  recessive).     The  formula  of  such  a  rabbit  would  be 

B2Br2E(R)ACI(D)U(S)Y2. 
This  indicates  a  heterozygous  condition  as  regards  5  character-units,  and 
rabbits  of  this  formula  should  be  capable  of  producing  32  different  gametic 
combinations,  16  of  which  would  be  visibly  expressed  in  different  pigmented 
varieties,  while  an  equal  number  lacking  the  factor  C  would  produce  albinos 
visibly  alike  but  gametically  different. 

If,  then,  we  were  to  carry  to  its  logical  conclusion  the  enumeration  of 
the  conceivable  different  varieties  of  gray  rabbit,  all  alike  in  appearance 
but  all  different  in  breeding  capacity,  i.  e.,  of  different  zygotic  formula, 
w^e  should  need  to  mention  32  varieties:  8  of  these  would  correspond  with 
the  first  8  which  have  already  been  enumerated  and  the  existence  of  which 
has  (except  for  variety  7)  been  demonstrated,  namely: 

(i)  Gray  producing  gray  only. 

(2)  Gray  producing  gray  and  black. 

(3)  Gray  producing  gray  and  white. 

(4)  Gray  producing  gray,  black,  and  white. 

(5)  Gray  producing  gray  and  yellow. 

(6)  Gray  producing  gray,  black,  yellow,  and  sooty. 

(7)  Gray  producing  gray,  yellow,  and  white. 

(8)  Gray  producing  gray,  black,  yellow,  sooty,  and  white. 

Eight  other  varieties  would  produce  the  same  sorts  of  3'oung  as  these  8, 
but  would  produce  in  addition  dilute  pigmented  ones  of  the  same  color 
types,  i.  g.,  blue-grays  as  well  as  grays,  blues  as  well  as  blacks,  pale  yellows 
(cream)  as  well  as  yellows,  and  pale  sooties  as  well  as  sooty  yellows. 

The  16  remaining  varieties  would  produce  the  same  sorts  of  young  as 
the  16  varieties  already  described,  but  would  produce  spotted  as  well  as 
uniformly  pigmented  (self)  individuals. 

Table  37. 


Color. 

Observed. 

Expected. 

Gray 

Black 

Yellow 

Sooty    

Blue-gray 

Blue 

Cream    

Pale  sooty 

24 
8 

16 
2 
8 
2 

3 
2 

27 
9 
9 
3 
9 
3 
3 
I 

COLOR 


59 


Not  every  one  of  these  32  varieties  of  gray  rabbit  has  actually  been 
demonstrated  to  exist  in  the  course  of  our  experiments;  10,  however,  which 
we  have  shown  to  exist,  have  already  been  mentioned,  and  several  others 
are  known.  For  example,  by  crosses  of  black  with  pale  yellow  (cream) 
or  of  blue  with  yellow,  we  have  obtained  grays  which  produced  the  same 
sorts  of  young  as  variety  6,  and  in  addition  blue-grays,  blues,  creams,  and 
pale  sooties.  Such  was  the  character  of  our  gray  females  1413,  1457,  '525> 
1526,  and  2009.  By  a  male  of  hke  character  they  have  produced  young 
as  shown  in  table  37. 

Other  gray  rabbits  producetl  by  the  same  cro.sses,  black  X  cream,  or 
blue  X  yellow,  produce  the  same  assortment  of  young,  and  in  addition 
albinos.  That  is,  they  are  hke  variety  8,  but  heterozygous  in  intensity  of 
pigmentation.  These  gray  rabbits,  females  1423,  1443,  and  1505,  and 
males  1351  and  1458,  mated  inter  se,  have  produced  young  as  indicated  in 
table  38. 

Table  38. 


Color. 

Observed. 

Expected. 

Gray 

Black 

Yellow 

Sooty    

Blue-gray 

Blue 

Cream    

Pale  sooty    

White 

20 
8 

13 
I 

7 

4 
(?) 

I 
8 

a? 
9 
9 
3 
9 
3 
3 
I 

31 

The  category  yellow  is  probably  too  large  because  of  a  failure  on  our 
part  to  discriminate  between  yellow  and  cream,  a  difference  which  at  t'lrst 
we  failed  to  record.  It  is  possible  also  that  albion  \oung  were  not  enu- 
merated in  all  the  records  which  we  have  combined,  and  so  albinos  are 
apparently  deficient  in  number. 

It  is  needless  to  go  farther  in  the  enumeration  of  zygotic  varieties  of  gray 
rabbits.  There  is  Uttle  doubt  that  the  entire  32  varieties  theoretically 
possible  coukl  readily  be  protluccd;  or  we  have  found  that  a  spotted 
coat  may  be  transferred  from  one  color  variety  to  another  by  means  of 
crosses,  and  the  same  is  true  of  a  dilute  condition  of  the  jjigmentation  in 
contrast  to  intense  pigmentation.  It  is  known  also  from  a  variety  of 
sources,  including  besides  our  own  observations  the  valuable  experiments 
of  Hurst  (."05),  that  albinism  may  occur  as  a  recessive  character  in  any 
and  all  color  varieties  of  rabbits.  Additional  evidence  seems  to  be  desir- 
able chiefly  as  concerns  the  assumed  factor  E;  therefore,  we  may  proceed 
to  the  consideration  of  color  varieties  other  than  gray,  in  the  course  of 
which  this  evidence  will  be  produced. 


60 


INHERITANCE    IN    RABBITS 


BLUE-GRAY. 

A  blue-gray  rabbit  differs  from  a  gray  one  only  in  the  intensity  of  its 
pigmentation,  which  is  always  dilute.  As  regards  the  intensity  factor, 
therefore,  it  is  invariably  homozygous,  D2,  since  D  is  recessive  to  I,  whereas 
a  gray  rabbit  may  be  either  homozygous,  1^,  or  heterozygous,  I  (D).  Con- 
sequently only  half  as  many  zygotic  combinations  are  possible  among  blue- 
gray  as  among  gray  rabbits,  16  instead  of  32  being  the  maximum. 

The  16  conceivable  varieties  of  blue-gray  rabbits,  all  of  which  should 
be  similar  in  appearance  but  different  in  breeding  capacity,  are: 

(i)  Blue-gray  producing  only  blue-gray;  formula,  B2Br2E2A2C2D2U2Y2. 

(2)  Blue-gray  producing  blue-gray,  and  blue;  formula,  B2Br2E2AC2D2U2Y2. 

(3)  Blue-gray  producing  blue-gray,  and  white;  formula,  B2Br2E2A2CD2U2Y2. 

(4)  Blue-gray  producing  blue-gray,  blue,  and  white;  formula,  B2Br2E2ACD2U2Y2. 

(5)  Blue-gray  producing  blue-gray,  and  cream;  formula,  B2Br2E(R)A2C2D2U2Y2. 

(6)  Blue-gray  producing  blue-gray,  blue,  cream,  and   pale  sooty;   formula, 

B2Br2E(R)AC2D2U2Y2. 

(7)  Blue-gray  producing  blue-gray,  cream,  and  white;  formula, 

B2Br2E(R)A2CD2U2Y2. 

(8)  Blue-gray  producing  blue-gray,  blue,  cream,  pale  sooty,  and  white;  formula, 

B2Br2E(R)ACD2U2Y2. 

The  8  remaining  varieties  would  be  identical  with  these,  except  for  the 
factor  U,  in  which  they  would  be  heterozygous,  U  (S),  producing  spotted 
as  well  as  self-pigmented  young. 

Three  blue-gray  rabbits,  all  females,  have  been  tested,  and  each  of 
these  is  of  a  different  zygotic  formula. 

Female  389,  the  original  blue-gray  individual,  proved  to  be  of  variety 
4.  When  mated  with  <?  248,  a  black  animal  heterozygous  in  E,  C,  and 
I,  i.  e.,  of  formula  B2Br2E(R)CI(D)U2Y2,  she  produced  gray,  blue-gray, 
blue,  and  white  young,  all  the  expected  classes  except  black  being  produced. 
The  observed  numbers  of  the  young  and  the  expected  proportions  are 
given  in  table  39. 

Table  39. 


Color. 

Observed. 

Expected. 

Gray 

Blue-gray  

Black 

Blue 

White 

4 

I 
0 

4 

I 

27 
9 
9 
3 

16 

Female  656  was  of  variety  2,  heterozygous  in  A  only,  as  is  shown  by 
table  40. 

Of  the  4  males  with  which  9  656  was  mated,  all  but  c?  1340  produced 
albino  young  in  other  matings.     This  indicates  clearly  that    9  656  was 


COLOR 


Gl 


not  heterozygous  in  C.      The  matings  with   males  402,   1340,  and   248 
show  that  she  was  homozygous  in  E. 

Table  40.  —  Matings  and  young  of  blue-gray  9  656. 


Mating. 

Gray. 

Blue- 
gray. 

Black. 

Blue. 

With  SOOtV  c^A020 

S 
1 

4 
.  0 

0 
0 
0 
3 

3 
1 

0 
0 

4 
0 

3 
3 

With  sootv  c^HA 

With  black  cJ'24» 

With  blue  (i'l  228 

Female  1437  was  either  of  variety  6  or  else  of  variety  8,  /.  e.,  she  was 
known  to  be  heterozygous  in  E  and  in  A,  but  was  insufficiently  tested  as 
regards  C.  Mated  with  blue  S  1434  she  produced  2  blue-gray  and  i  pale 
sooty  young  and  i  pigmented  animal  of  uncertain  character. 


BLACK. 

Black  rabbits,  as  we  have  already  ob.served,  differ  from  gray  ones  only 
in  the  factor  A,  which  they  lack  complete!}.  16  different  zygotic  com- 
binations are  theoretically  possible  among  them. 

(i)  Black  producing  nothing  but   black;   formula,  B-^BrjEjCjIjUxYj  (compare 
diagram,  p.  52). 

(2)  Black  producing  black,  and  white;  formula,  B2Br,E2CI,U2Y2. 

(3)  Black  producing  black,  and  sooty;  formula,  B2Br,E(R)C,l2U2Y2. 

(4)  Black  producing  black,  sooty,  and  white;  formula,  B.^Br2E(R)Cl2U2Y2. 

(5)  Black  producing  black,  and  blue;  formula,  B2Br2E2C2l (DjUj Yj. 

(6)  Black  producing  black,  blue,  and  white;  formula,  B2Br2p>2d(D)U2Y2. 

(7)  Black  producing  black,  blue,  sooty,  and  pale  sooty;  formula, 

B2Br2E(R)C2l(D)U2Y2. 

(8)  Black  producing  black,  blue,  sooty,  pale  sootv,  and  while;  ft)rmula, 

B2Br2E(R)CI(D)U2Y2. 

The  8  remaining  varieties  would  be  identical  with  these  8,  except  that 
they  would  be  heterozygous  as  regards  U,  viz,  U(S)  instead  of  U.  Con- 
sequently they  would  produce  spotted  as  well  as  self-pigmented  young. 

All  the  black  rabbits  (with  one  exception)  which  we  have  used  for  breed- 
ing purposes  were  produced  in  the  course  of  our  experiments  from  animals 
of  other  color  varieties.  All  were  in  one  or  more  respects  heterozygous, 
except  possibly  the  recently  purchased  black  rabbit,  not  yet  full\  tested, 
but  apparently  homozygous  in  all  particulars  and  so  of  variety  i.  Hurst 
(:o5)  obtained  rabbits  of  variety  2,  but  we  do  not  hajjpen  to  have  hail  any 
of  this  variety,  nor  of  variety  3. 

Variety  4  is  re])resented  in  a  modified  form  in  our  rabbits  104,  105,  167, 
247,  and  255.  The  modification  consists  in  this:  the  albino  otTs|)ring  arc, 
at  least  in  part,  of  the  HimalaNan  type,  having  pigmented  extremities. 


62 


INHERITANCE    IN   RABBITS 


This  we  might  express  by  adding  the  Himalayan  factor  (C)  to  the  for- 
mula as  given  for  variety  4.  Variety  5  is  represented  in  our  black  X, 
which  when  mated  with  blue  c?  1434  (variety  3)  produced  4  black  and 
4  blue  young. 

Variety  7  is  represented  in  our  rabbits  1230  and  1231,  201 1,  and  2038; 
and  variety  8,  in  a  modified  form,  in  our  d  248,  which  has  sired  black,  blue, 
sooty,  pale  sooty,  white,  and  Himalayan  w^hite  offspring  by  black,  sooty- 
yellow,  or  blue-gray  mates.  He  therefore  differs  from  variety  8  as  previ- 
ously described  in  that  he  is  heterozygous  in  the  Himalayan  factor  C 
His  formula  accordingly  is  B2Br2E(R)C'(C)I(D)U2Y2.  Some  evidence  for 
this  classification  of  our  black  animals  will  be  found  in  the  table  41.  Other 
evidence  is  derived  from  matings  with  yellow  or  gray  animals. 

Table  41.  — Matings  of  black  rabbits  with  black  or  sooty  individuals. 


Mating. 

Black. 

Sooty. 

Hima- 
layan. 

White. 

Blue. 

Pale 
sooty. 

$  105  black  with  c?io4  black 

9  167  black  with  c?248  black 

9  247  black  with  (^248  black 

9  25s  black  with  0^248  black 

Total      

9 
2 

3 
10 

I 

3 
2 

I 

I 

I 
2 

2 

24 
9 

7 
3 

4 
3 

2 

I 

Expected  proportions 

9 1230  black  with  c?i340  sooty 

Expected    

3 
I 

4 

3 

4 

I 

4 
3 

E 

;::; 

I 
I 

■  V 

9 1230  black  with  (^1414  sooty 

Expected    

BLUE. 

"Blue"  pigmentation  in  rabbits  and  other  rodents  is  merely  a  dilute 
condition  of  black.  The  zygotic  formula  of  a  blue  rabbit  is  the  same  as 
that  of  a  black  one,  if  we  substitute  D2  for  the  I2  or  1(D)  of  the  black  vari- 
eties.    Blue  rabbits  may  occur  theoretically  of  8  dift'erent  sorts,  viz: 

(i)  Blue  producing  blue  only;  formula,  B2Br2E2C2D2U2Y2. 

(2)  Blue  producing  blue,  and  white;  formula,  B2Br2E2CD2U2Y2. 

(3)  Blue  producing  blue,  and  pale  sooty;  formula,  B2Br2E(R)C2D2U2Y2. 

(4)  Blue  producing  blue,  pale  sooty,  and  white;  formula,  B2Br2E(R)CD2U2Y2. 


Table  42.  —  Matings  and  young  of  S  1434,  blue. 


Mating. 


With  9647,  sooty,  variety  2 

Expected 

With  $1471,  sooty,  variety  3  or  4. 

Expected 

With  9  black,  variety  5  or  6 

Expected 


Black. 


Blue. 


Sooty. 


Pale 
sooty. 


CO  LOU 


63 


The  4  remaining  varieties  would  be  identical  witii  these  4  except  as 
regards  the  factor  U,  in  which  they  would  be  heterozygous,  U(S),  instead 
of  homozygous,  Uj. 

We  have  determined  the  zygotic  formuke  of  2  blue  rabbits  only,  botii 
of  which  were  produced  in  the  course  of  our  experiments.  One  (s  1434, 
table  42)  was  of  variety  3,  the  other  (S  1228,  table  43)  was  of  variety  4. 

We  shall  pass  by  the  spotted  black  and  spotted  blue  varieties  of  rabbit, 
of  both  which  sorts  a  certain  number  of  individuals  have  been  jiroduced 
in  our  experiments,  but  wliich  have  not  been  thoroughly  tested. 

Table  43.  —  Malings  and  young  of  cf  1228,  blue. 


Mating. 


With  9647,  sooty,  variety  2 

Expected    

With  9  1280,  sooty,  variety  3  or  4 

Expected    

With  9656,  blue-gray,  variety  a  . . 

Expected    , 


Blue- 
gray. 


Black. 


IS 
3 

2 
I 


Blue. 


Sooty. 


Pale 
sooty. 


White. 


17 
3 

2 
I 


o 
I 


YELLOW. 

Yellow  rabbits  differ  from  gray  ones  only  in  the  factor  E  (extended 
black  or  brown  pigmentation),  in  place  of  which  they  bear  the  alternative 
condition  R  (restricted  black  or  brown  pigmentation).  Since  R  is  reces- 
sive in  relation  to  E,  yellow  rabbits  are  invariably  homozygous,  R,,  as 
regards  this  factor.  Theoretically  sixteen  different  varieties  are  possible, 
as  follows: 

(i)  Yellow  producing  yellow  only;  formula,  BjBrjRoAjCoIjUjYj. 

(2)  Yellow  producing  yellow,  and  wliite;  formula,  BoBtjRj.VjCIjUjYj. 

(3)  Yellow  producing  yellow,  and  sooty;  formula,  BiBrjRoACoIjUjYj. 

(4)  Yellow  producing  yellow,  sooty,  and  white;  formula,  BiBtjRo-^CIjUjYj. 

Four  other  varieties  should  differ  from  these  4  in  factor  I  only,  being 
1(D)  instead  of  I2,  and  producing  dilute  as  well  as  intensely  jiigmcnted 
individuals.  Eight  others  should  differ  from  these  eight  in  producing 
spotted  as  well  as  uniformly  pigmented  individuals.  We  shall  content 
ourselves  with  giving  examjiles  of  the  first  four  varieties  enumerated. 

Variety  i  produces  only  yellows  when  mated  with  other  yellows  or 
with  sooties,  and  only  yellows  and  grays  when  mated  with  blacks  or  blues 
of  any  sort  whatever.  It  is  represented  in  our  yellow  (^381,  a  son  of  2 
gray  rabbits  of  variety  5,  viz,  9  175,  and  cJiyC.  He  was  mated  with  6 
different  yellow  females,  3  of  which  had  produced  sooty  offspring  by  other 
mates,  and  there  resulted  61  young,  all  yellow.  In  matings  with  2  dilTer- 
ent  sooty  females  he  produced  12  young,  all  yellow;  and  in  a  mating 
with  a  gray  female  of  variety  6  (9  178)  he  produced  3  yellow  and  6  gray 


64  INHERITANCE    IN    RABBITS 

young;  expected  i:  i.  We  have  had  several  other  yellow  rabbits  which 
were  probably  of  this  same  variety,  but  they  were  less  extensively  and 
inconclusively  tested. 

Variety  2  is  represented  in  a  yellow  rabbit  obtained  by  purchase 
(c?i256).  He  was  mated  with  9  547  yellow,  variety  3,  and  produced 
9  young,  all  yellow  (as  expected) ;  by  yellow  9  745^,  variety  4,  he  produced 
4  yellow  and  3  white  young  (expected  3:1);  and  by  black  9  1230  and 
9  1 23 1,  variety  7,  he  produced  8  yellow,  3  gray,  and  i  blue-gray  young; 
expected  4:  3:  i. 

Variety  3  is  represented  in  yellow  females  547,  714,  and  11 15,  which 
in  matings  with  yellow  males  of  variety  4  produced  24  yellow  and  2  sooty 
young,  but  no  white  ones.  Had  these  females  been  of  variety  4  they 
should  have  produced  25  per  cent  of  white  young  in  the  mating  mentioned. 
It  is  possible  that  the  recorded  number  of  sooties  is  too  small,  owing  to 
a  failure  in  our  earUer  records  to  discriminate  sooty  from  yellow.  No 
such  possibility  exists  in  the  case  of  the  records  for  albinos.  One  of  the 
females  already  mentioned,  of  variety  3  (9  1115),  when  mated  with  sooty 
J  1340  produced  2  yellow  and  6  sooty  yoimg. 

Another  j^ellow  rabbit  of  variety  3  was  the  lop  cf  179  (plate  2,  fig.  8). 
When  mated  with  the  sooty  "old  female  lop"  (plate  i,  fig.  2)  he  produced 
4  yellow  and  4  sooty  young  (expected  1:1);  and  when  mated  with  black 
females  of  variety  4  (9  9  105,  167,  and  247)  he  produced  7  gray,  5  black, 

6  yellow,  and  7  sooty  young  (expected  1:1:1:1).  Notice  in  the  matings 
with  black  females  the  total  absence  of  albinos,  though  all  these  females 
had  produced  albinos  by  other  mates. 

Still  another  male  of  variety  3,  c?3i9,  son  of  the  sooty  old  female  lop 
by  gray  J  176,  variety  5,  when  mated  with  the  black  9  247  (variety  4) 
produced  2  gray,  2  black,  2  yellow,  and  i  sooty  young  (expected  i:  i:  i:  i). 

Variety  4  is  represented  in  our  "Cutler's  yellow"  and  in  9  745^  pro- 
duced by  black  9105  (variety  4)  mated  with  yellow  c?3i9  (variety  3). 
When  "Cutler's  yellow"  was  mated  with  the  above  female,  745-2-,  he  pro- 
duced 4  yellow,  3  sooty,  and  i  white  young  (expected  9:3:4),  WTien 
mated  with  sooty  females  632  and  647  (variety  2),  he  produced  7  yellow, 

7  sooty,  and  2  white  young  (expected  3:3:2). 

SOOTY. 

Sooty  rabbits  differ  from  yellow  ones  only  in  the  factor  A,  which  they 
lack.     Theoretically  8  varieties  are  possible,  viz: 

(i)  Sooties  producing  sooties  only,  when  mated  inter  se;  formula,  B2Br2R2C2l2U2Y2. 

(2)  Sooties  producing  sooty,  and  white;  formula,  B2Br2R2Cl2U2Y2. 

(3)  Sooties  producing  sooty,  and  pale  sooty;  formula,  B2Br2R2C2l(I^)U2Y2. 

(4)  Sooties  producing  sooty,  pale  sooty,  and  white;  formula,  B2Br2R2CI(D)U2Y2. 

The  4  remaining  varieties  would  be  like  these,  except  as  regards  the 
factor  U,  in  which  they  would  be  heterozygous,  U(S),  instead  of  homozy- 


COLOR  65 

gous,  U3.      They  would    produce  spotted  as  well  as  uniformly  j^igmented 
young. 

An  exam|)lc'  of  variety  i  is  the  "old  female  lop"  (plate  i,  fig.  2).  When 
mated  with  an  albino  of  black  ancestry,  6  45  ([)late  i,  fig.  3),  she  producccl 
a  litter  of  8  black  young  (plate  i,  fig.  i).  This  experiment  shows  her  to 
have  been  homozygous  in  C,  /.  e.,  to  have  been  Cj  in  character,  anfl  to 
have  lacked  factor  A.  When  mated  with  yellow  .i  179  (plate  2,  fig.  8), 
variety  3,  she  jjroduced  4  yellow  and  4  sooty  young,  exactly  the  exi>ect«l 
equaUty  of  yellow  and  sooty.  Another  individual  probably  of  this  same 
variety  was  9  1472,  which  when  mated  with  a  sooty  male,  1414,  produce<l 
12  young,  all  sooty.  The  male  just  mentioned  belonged  apparently  to 
variety  3,  for  when  mated  willi  the  black  9  1230  (variety  7)  he  j)roducefl 
5  black,  6  sooty,  and  i  blue  young  (expected  3:4:1;  or  if  pale  sootics 
v^Tre  differentiated  from  sooties,  which  we  probably  failed  to  fio  in  making 
the  records,  3  black  :  3  sooty  :  i  blue  :  i  pale  sooty). 

Variety  2  is  represented  in  our  j"  402  and  99632  and  647.  When 
c?  402  was  mated  with  9632,  they  produced  a  litter  of  4  sooty  and  i  white 
young  (expected  3:  i).  Female  647,  when  mated  with  "Cutler's  yellow" 
(variety  4),  produced  5  yellow,  4  sooty,  and  i  white  young  (expecte<l 
3:3:2).  The  white  individual  ]>roduced  by  9632  and  cf  402  was  a 
Himalayan  albino.  This  shows  one  or  both  of  the  parents  to  have  been 
slightly  different  from  typical  variety  2,  and  to  have  carried  C. 

Variety  3  is  represented  probably  by  our  9  147 1  which,  when  mated 
with  blue  d*  1434,  produced  4  black,  2  blue,  5  sooty,  and  2  pale  sooty  young 
(expected  1:1:1:1).  The  possibility  is  not  excluded  that  this  female 
was  of  variety  4  (capable  of  producing  also  albino  young),  but  she  can  not 
have  been  of  either  variety  i  or  variety  2.  Another  j^robable  example  of 
variety  3  is  9  1280.     (See  matings  of  d*  1228,  blue,  p.  63). 

Variety  4  we  have  not  identified  with  certainty.  Neither  have  we 
made  a  detailed  study  of  pale  yellows,  pale  sooties,  or  spotted  rabbit.s  of 
any  color  variety.  We  have  observed,  liowever,  that  dilute  pigmentation, 
as  well  as  spotting,  occurs  in  all  the  fundamental  color  varieties  and  are 
entirely  satisfied  of  the  independent  inheritance  of  both. 

WHITE. 

Albino  rabbits  differ  from  i)igmented  ones  onl\-  in  regard  to  the  factor 
C,  which  they  either  lack,  or  possess  only  in  a  greatly  modified  form,  C. 
If  C  is  absent,  there  are  jiossible  i6  dilTerent  combinations  of  the  4  remain- 
ing variable  factors,  which  combinations  corresj)ond  with  gametes  of  the 
16  visibly  different  i)igmented  varieties  of  rabbit,  minus  C.  But  if  C  is 
present  in  the  modified  form,  C,  found  in  Himalayan  albinos.  16  other 
gametic  combinations  should  be  ])Ossible,  only  slightly  difierent  from  the 
foregoing,  making  in  all  32  difierent  gametic  possibilities,  or  232  zygotic 
possibilities. 


66  INHERITANCE    IN   RABBITS 

Plainly  it  is  unprofitable  to  attempt  to  find  illustrations  of  all  these  con- 
ceivable variations.  We  shall  content  ourselves  with  noticing  some  of 
the  more  important  varieties  of  albinos  and  presenting  evidence  that  each 
of  the  4  variable  factors,  A,  E,  I,  and  U,  is  transmitted  through  albinos. 

The  foUovv^ing  albino  varieties  may  be  expected  to  occur: 

(i)  White  producing  gray  only  (in  crosses  with  any  pigmented  variety);  for- 
mula, BjBrjEjAjIjUjYj. 

(2)  White  producing  black  only  (in  crosses  with  black  or  any  pigmented  variety 

recessive  to  black);  formula,  BjErjEjIjUaYj. 

(3)  White  producing  yellow  only  (in  crosses  with  yellow  or  sooty  individuals) ; 

formula,  BjBrjRjAjIjUjYg. 

(4)  White  producing  sooty  only  (in  crosses  with  sooty) ;  formula,  BoBrjRjIzUjYj. 

(5)  White  producing  gray,  and  black  (in  crosses  with  black  or  any  pigmented 

variety  recessive  to  black);  formula,  BjBrjEjATjUjYj. 

(6)  White  producing  gray,  and  yellow  (in  crosses  with  yellow  or  sooty) ;  formula, 

B2Br3E(R)A2l2U2Y2. 

(7)  White  producing  gray,   black,   yellow,   and  sooty  (in  crosses  with  sooty) ; 

formula,  B2Br2E(R)Al2U,Y2. 

(8)  White  producing  black,  and  sooty  (in  crosses  with  sooty);  formula, 

B2Br2E(R)l2U2Y2. 

(9)  White  producing  yellow,  and  sooty  (in  crosses  with  sooty);  formula, 

B2Br2R2Al2U2Y2. 

Another  set  of  9  varieties,  quite  similar  to  these,  w^ould  produce  only 
pale-pigmented  offspring.  As  regards  the  intensity  factor  they  would 
be  D2  instead  of  I2.  Another  set  of  9  varieties  would  produce  both  dilute 
and  intensely  pigmented  offspring,  being  heterozygous,  1(D),  as  regards 
the  intensitv  factor. 

Nine  other  varieties,  in  which  S2  replaces  U2,  would  produce  only  spotted 
young;  and  another  set  of  9  would  produce  both  spotted  and  self-colored 
offspring;  in  these  U(S)  would  replace  U2.  Another  set  of  9  varieties 
would  produce  only  pale-pigmented  spotted  individuals,  another  would 
produce  pale-pigmented  individuals,  both  self  and  spotted;  and  lastly  a 
set  of  9  varieties  would  produce  both  dilute  and  strongly  pigmented  indi- 
viduals, both  spotted  and  self-colored. 

It  is  probable  that  the  foregoing  list  of  72  varieties  could  be  duplicated 
in  varieties  having  the  Himalayan  modification,  and  duplicated  a  second 
time  in  varieties  heterozygous  in  the  two  sorts  of  albinism. 

A  few  examples  will  now  be  mentioned  of  some  of  the  9  varieties  of 
albinos  first  enumerated,  or  of  animals  differing  from  those  9  varieties 
in  one  or  two  characters  only. 

Variety  i  is  represented  in  our   s  1425,  which  when  mated  with  black 

9  1 541  produced  11  young,  all  gray,  and  when  mated  with  yellow  9  547 

(variety  3)  produced  4  young,  all  gray.     Variety  2,  but  heterozygous  in 

the  Himalayan  modification,  C,  and  in  spotting  with  white,  U(S),  is  rep- 


COLOR  67 

resented  in  our  (^45  (plate  i,  fig.  t,),  which  when  mated  with  the  sooty 
lop  (plate  I,  fig.  2)  produced  8  young,  all  black.  One  of  these  is  shown 
in  plate  i,  fig.  1.  When  mated  with  black  9  105,  he  proflucetl  8  black  jiig- 
mented  and  3  Himalayan  albino  young,  but  .several  of  the  {iigmtntcnl 
young  were  spotted  with  white,  this  character  being  recessive  in  9  105, 
which  had  a  Dutch-marked  father. 

Variety  5  is  represented  in  9  269,  which  when  mated  with  sooty  d"  402 
produced  i  gray  and  3  black  young  (expected  1:1).  When  mate<l  with 
yellow  d"  179  (variety  3)  she  produced  in  one  litter  3  black  young,  and  in 
a  second  litter,  3  gray  and  i  black.  This  second  litter,  in  which  the 
expected  proj)ortions  of  gray  and  of  black  young  are  exactly  realizerl,  is 
shown  in  plate  2,  fig.  6,  the  parents  being  shown  in  figs.  5  and  8  of  the  same 
plate. 

Variety  8  is  probably  represented  in  9  268  which  when  mated  with  yellow 

<?3i9  (variety  3)  produced  2  gray,  i  yellow,  2  black,  and  2  sooty  young 

(expected  1:1:1:1).     The  only  other  possibility  is  that  this  female  was 

of  variety  7,  which  should  in  this  mating  produce  the  same  varieties  oi 

young,  but  in  the  proportions,  9:3:3:1. 

Variety  8  (but  heterozygous  in  C,  the  Himalayan  factor)  is  represented 
in  9  108,  which  when  mated  with  black  c?  104  (variety  4)  produced  3 
black,  3  sooty,  and  6  Himalayan  albino  young,  exactly  the  expected  pro- 
portions. By  yellow  <?i79  (variety  3)  she  produced  i  gray,  i  yellow, 
and  I  sooty  young  (expected  1:1:1:1  black) . 

The  foregoing  cases  would  afford  confirmation  (if  confirmation  were 
necessary)  of  the  discovery  by  Cudnot  (:o3)  and  by  Hurst  (:o5)  that  albino 
mammals  transmit  color  factors,  and  that  they  vary  in  zygotic  composi- 
tion as  regards  color  factors.  That  albinos  transmit  the  factor  A  is  shown 
by  the  observation  that  some  of  them  (which  bear  A)  |>roduce  gray  otT- 
spring  in  crosses  with  black  j)igmented  animals,  while  others  (lacking  A) 
never  produce  gray  offspring,  though  mated  to  the  same  black  animals. 

That  albinos  transmit  the  factor  E  is  shown  clearly  by  extensive  ex|x>ri- 
ments  with  guinea-pigs  carried  out  by  one  of  us.  One  family  of  albino 
guinea-pigs  has  been  found  invariably  to  produce  black  offsi>ring  in  mat- 
ings  with  any  pigmented  variety  devoid  of  factor  A,  whether  that  varirty 
has  the  extended  or  the  restricted  distribution  of  black  or  brown  pigment; 
a  second  family  of  guinea-pigs,  with  equal  uniformity,  produces  colortd 
offspring  having  a  restricted  distribution  of  black  pigment,  if  cro.ssed  with 
colored  individuals  having  the  restricted  distribution.  This  second  vari- 
ety produces  black-eyed  yellows,  if  crossed  either  with  black-eyed  yellow 
or  with  brown-eyed  yellow  individuals.  Of  the  2  albino  varieties  men- 
tioned, the  first  evidently  carries  B  with  E,  the  second  B  with  R. 

These  same  two  famihes  of  albino  guinea-pigs  likewise  ditTer  in  factor 
I,  which  is  present  in  the  first  family,  but  rej^laced  by  D  in  the  second. 
If  each  is  crossed  with  pale  yellow  (cream)  individuals,  the  former  produces 


68  INHERITANCE   IN    RABBITS 

in  Fj  black  offspring,  and  in  Fj  black,  blue,  red,  and  cream,  as  well  as 
albinos;  whereas  the  latter  produces  in  Fj  cream,  and  in  F2  cream  and 
albino  offspring  only. 

As  regards  the  factor  U,  Hurst  ( :  05)  has  shown  clearly  that  some  albino 
rabbits  transmit  a  uniformly  colored  coat,  others  a  spotted  coat,  in  crosses 
with  colored  rabbits.  The  former  we  may  regard  as  carrying  U,  the  latter 
S.  In  rabbits  we  have  not  made  an  extensive  study  of  this  matter.  We 
have  found,  however,  in  agreement  with  Hurst,  and  Woods  (:o3)  that 
spotted  rabbits  in  general  produce  only  spotted  young,  when  mated  with 
each  other,  i.  e.,  that  spotting  with  white  is  recessive  to  uniform  pigmenta- 
tion, and  the  case  has  been  mentioned  of  an  albino  (c?  45)  which  produced 
spotted  young  when  mated  with  a  black  rabbit  that  had  a  spotted  father. 

In  guinea-pigs  also,  spotting  is  in  the  main  recessive,  and  spotting  is 
clearly  transmitted  by  albinos.  Thus  the  <S  2002,  figured  and  described 
by  Castle  (:  05),  produced  spotted  young  when  mated  with  spotted  females, 
and  among  his  grandchildren  spotted  animals  were  very  common,  no  matter 
whether  the  female  grandparent  and  the  parents  were  spotted  or  not. 

All  the  varied  evidence  which  has  been  obtained  from  the  study  of 
rabbits,  guinea-pigs,  rats,  and  mice,  by  others  as  well  as  by  ourselves, 
supports  the  hypothesis  that  albinos  differ  from  pigmented  individuals, 
by  a  single  factor  only,  which  factor  we  call  C. 

THE   MATERIAL   BASIS   OF   HEREDITY   FACTORS, 

In  what  form,  it  may  be  asked,  are  we  to  suppose  that  the  various 
assumed  factors  exist.  Do  they  occur  as  so  many  different  substances 
lying  side  by  side  but  unmixed  in  every  reproductive  cell?  To  this  ques- 
tion we  may  give  at  present  no  satisfactory  answer. 

It  is,  however,  we  think,  not  necessary  to  suppose  that  there  exist  in 
the  minute  germ-cell  as  many  complex  organic  substances  as  there  are 
activities  of  the  cell;  neither  is  it  necessary  to  suppose  a  different  substance 
present  for  every  independent  factor  identified.  The  various  indepen- 
dent factors  may  have  a  basis  no  more  complicated  than  that  of  so  many 
atoms  attached  to  a  complex  molecular  structure.  Experiment  shows  that 
the  factors  may  be  detached  one  by  one  from  the  organic  complex.  The 
discontinuity  of  their  coming  and  going  is  entirely  in  harmony  with  the 
conception  of  them  as  components  merely  of  complex  molecular  bodies. 


BIBLIOGRAPHY. 


Bateson,  W. 

:o6.    The  progress  of  Genetics  since  the  rediscovery  of   Mendel's  papers.     Progressus 
rei  botanicae,  i,  pp.  368-418. 
Bateson,  W.,  Saunders,  E.  R.,  and  Punnett,  K.  C. 

:o6.     Experimental  studies  in  the  physiology  of  heredity.     Rcp<jrt  III  to  the  Evolution 
Committee  of  tlie  Royal  Society.     53  pp.     London. 
Castle,  VV.  E. 

:  05.     Heredity  of   coat    characters  in    guinea-pigs    and    rahhits.     Publication   No.    aj 

of  the  Carnegie  Institution  of  Washington,  78  pp.,  6  pi. 
:o5<i.  Recent  discoveries   in    heredity  and  their  bearing  on  animal  breeding.     Popular 

Science  Monthly,  vol.  66,  pp.  193-208,  14  fig.,  July  1905. 
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vol.  25,  pp.  151-153- 
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Science,  n.s.,  vol.  26,  pp.  287-291. 
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Castle,  W.  E.,  and  Forbes,  A. 

:  06.     Heredity  of  hair-length  in  guinea-pigs  and  its  bearing  on  the  theory  of  pure  gam- 
etes.     Publication    No.    49   of    the    Carnegie    Institution    of    Washington, 
pp.  1-14. 
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103.     L'h^r^dite    de  la  pigmentation   chez    les   souris.     (2™*  Note.)     Arch,  de   Zool. 

exper.  et  g6n.  (4),  tome  i,  Notes  et  revue,  pp.  33-41. 
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body.     Boas  Memorial  Volume,  New  York.     pp.  5-26,  2  pi. 
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Harvard  Univ.,  vol.  3,  No.  3,  pp.  69-77,  p'-  23-27. 
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107.     Variations   e.xperimentales.     Etudes    sur  six  generations    de   poules  carnivores. 
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Roy.  Bot.  Gardens,  Peradeniya,  vol.  3,  pt.  2,  pp.  95-184. 
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TSCHERMAK,    E. 

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Woods,  F.  A. 

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69 


DESCRIPTION    OF    PLATES. 


Plate  i  .  —  Photographs  from  life  of  rabbits  described  in  the  text. 

Fig.  I.  (5^248,  son  of  the  rabbits  shown  in  figs.  2  and  3;  ears  of  intermediate  length,  hair 
short,  color  black. 

2.    "  Old  female  lop,"  sooty  yellow  in  color. 

3-    C?45)  ^  short-eared,  Himalayan  albino,  angora  rabbit. 

4.    9  400,  daughter  of  c?  248,  fig.  i,  and  of  his  sister  9  247,  a  rabbit  of  like  character. 

Plate  2.  —  Photographs  from  life  of  rabbits  described  in  the  text. 
Fig.  5.   9  269,  a  short-eared  albino  rabbit. 

6.  A  litter  of  four  rabbits  (640  to  643)  borne  by  9  269,  fig.  5,  when  mated  with  c?i79, 

fig.  8.     Three  are  gray,  one  is  black;  all  have  ears  of  intermediate  length, 
as  compared  with  the  parents. 

7.  Gray  quarter-blood  lops  borne  by  9  43i,  plate  3,  fig.  9,  in  a  mating  with  her  son 

C?  176,  a  half-blood  lop  similar  in  appearance  to  his  sister  9  I75)  plate  3,  fig.  10. 

8.  c?i79,  a  full-blood  yellow  lop,  son  of  the  "old  female  lop,"  plate  i,  fig.  2,  and  of  the 

"old  male  lop,"  a  yellow  rabbit. 

Plate  3.  —  Photographs  from  life  {except  fig.  9)  of  rabbits  described  in  the  text. 
Fig.  9.  Mounted  skin  of  9  431.  the  "Belgian  hare,"  a  gray  rabbit  with  short  ears. 

ID.   9  175)  ^  gray  half-blood  lop,  daughter  of  9  43i>  %•  9,  and  the  old  (J*  lop,  a  yellow 
rabbit  similar  in  appearance  to  his  son  (^179,  plate  2,  fig.  8. 

11.  9322,  a  gray  three-quarter  blood  lop,  daughter  of  old  female  lop,  plate  i,  fig.  2, 

and  the  half-blood  lop  (^176;  compare  fig.  10,  which  gives  a  good  idea  of 
the  appearance  of  (J*  176. 

12.  c?38i,  son  of  9  175,  fig-  10,  and  her  brother,  c?i76;  an   F2  half-blood  lop,  with   the 

same  general  ear-character  as  his  parents,  but  yellow  in  color,  like  his  grand- 
father. 

Plate   4.  —  Dorsal  and  ventral  views  of  the  skulls  of  3  rabbits. 

In  the  middle  the  skull  of  c?248  (compare  plate  i,  fig.  i);  at  the  right  the  skull  of  his  mother 
"old  female  lop"  (plate  i,  fig.  2);  and  at  the  left  the  skull  of  his  father 
6^45  (plate  i,  fig.  3). 

70 


CASTLE 


PLATE      i 


CASTLE 


PLATE    2 


■-^-iw*-^ 


CASTLE 


PLATE  3 


•<i.vif 


CASTLE 


PLATE    4 


N.  C,  btiiLeCoUe  -e 


